Recent converts to Judaism were 'of course' seen as extreme traitors to begin with.
However, Nazi ideology was based primarily on 'blood' and race, not so much on secondary attributes like beliefs or 'faith'. As such a German/'Aryan' converting to Judiasm would still be carrying 'Aryan blood'. The Nuremberg Race laws themselves make indeed no direct provisions for proselytes, ancestry was all that's considered in them, and exact definition were lacking.
That does not mean that obedient racists in jurisprudence and politics did not see this extremely tiny number in the population as 'a problem to be dealt with'.
The direct commentary on the Nuremberg Laws by later federal West-German state secretary Globke and Wannsee conference member Stuckart details how converts were to be subjected under the Nuremberg Laws:
Direct converts to Judaism could escape persecution by revoking their conversion. However, this did not apply to descendants of converts who were considered by the National Socialists to be Germans of Jewish origin.
"Belonging to the Jewish religious community" [was] "generally regarded as such a strong commitment to Judaism […] that the passing on of the Jewish attitude to the descendants had to be expected."
Hans Globke and Wilhelm Stuckart thus justified the inclusion of converts in the Nuremberg racial legislation.
— Wilhelm Stuckart & Hans Globke: "Kommentare zur deutschen Rassengesetzgebung" 1936" (Reichsbürgergesetz vom 15. September 1935: Gesetz zum Schutze des deutschen Blutes und der deutschen Ehre vom 15. September 1935; Gesetz zum Schutze der Erbgesundheit des deutschen Volkes (Ehegesundheitsgesetz) vom 18. Oktober 1935; Nebst allen Ausführungsvorschriften u. d. einschläg. Gesetzen u. Verordnungen), C. H. Beck: München, Berlin, 1936. p 64. dnb
As already stated, within the Reich, the number of people directly affected by this was very small, and this is not saying anything about how people of this definition that would be encountered in occupied lands during the war were to be treated. That is: often even worse if 'found out' (no options given).
As Saul Frieländer highlighted:
The main commentary on the “German racial legislation,” published that same year, was coauthored by Secretary of State (in the Ministry of the Interior) Wilhelm Stuckart, and another official from the same ministry, Hans Globke, whose passion for identifying Jews by their names will be encountered later. It reveals starkly some of the most perplexing aspects — even from the Nazi viewpoint — of the Nuremberg Laws. In order to illustrate the absolute validity of religious affiliation as the criterion for identifying the race of the descendants, Stuckart and Globke gave the hypothetical example of a woman, fully German by blood, who had married a Jew and converted to Judaism and then, having been widowed, returned to Christianity and married a man fully German by blood. A grandchild deriving from this second marriage would, according to the law, be considered partly Jewish because of the grandmother’s one-time religious affiliation as a Jew. Stuckart and Globke could not but state the following corollary:
“Attention has to be given to the fact that…[in] terms of racial belonging, a full-blooded German who converted to Judaism is to be considered as German-blooded after that conversion as before it; but in terms of the racial belonging of his grandchildren, he is to be considered a full Jew.”
The racial mutation caused by such temporary contact with the Jewish religion is mysterious enough. But the mystery is compounded when it is remembered that in Nazi eugenics or racial anthropology, the impact of environmental factors was considered negligible in comparison with the effect of heredity. Here, however, an ephemeral change in environment mysteriously causes the most lasting biological transformation.
But whatever their origins, racial differences could lead to dire consequences in cases of prolonged mixing:
“The addition of foreign blood to one’s own brings about damaging changes in the body of the race because the homogeneity, the instinctively certain will of the body, is thereby weakened; in its stead an uncertain, hesitating attitude appears in all decisive life situations, an overestimation of the intellect and a spiritual splitting. A blood mixture does not achieve a uniform fusion of two races foreign to each other but leads in general to a disturbance in the spiritual equilibrium of the receiving part.”
— Saul Friedländer: "Nazi Germany and the Jews, Volume I, The Years of Persecution, 1933–1939", HarperCollins: New York, 2008. p152.
The most central law then for 'identifying/categorising' offspring is an ordinance that was intended to clear up how to handle the citizenship status in the Reich: "Erste Verordnung zum Reichsbürgergesetz, 14.11.1935, RGBlI 1935, S. 1333f", defining the terms "Jew" and "Jewish half-breed", also containing the one-time religious affiliation as defining biological race as Friedländer explained:
"A Jewish half-breed is anyone who is descended from one or two grandparents who are fully Jewish by race, unless they are considered Jewish in accordance with § 5 Paragraph 2. A grandparent is considered fully Jewish without further ado if he or she belonged to the Jewish religious community. "Section 5 defines who is Jewish, or is considered Jewish: Paragraph 1: "A Jew is one who is descended from at least three grandparents who are fully Jewish by race." Paragraph 2: "A Jew shall also be deemed to be a mixed-blood Jewish citizen descended from two fully Jewish grandparents, a) who belonged to the Jewish religious community at the time of the enactment of the law or is subsequently admitted to it, b) who was married to a Jew at the time of the enactment of the law or is subsequently married to such a person,c) who is descended from a marriage to a Jew within the meaning of paragraph 1, which took place after the entry into force of the Law for the Protection of German Blood and German Honor (cf. RGBl I 1935, p. 1146),d) who comes from extramarital intercourse with a Jew within the meaning of paragraph 1 and is born out of wedlock after July 31, 1936. "Section 4 stipulates: "A Jew may not be a citizen of the Reich. He shall not be entitled to vote in political matters; he may not hold public office."
— Erste Verordnung zum Reichsbürgergesetz, Materialien zum Nationalsozialismus — NS-Quellen.at translated
This "A grandparent is considered fully Jewish without further ado if he or she belonged to the Jewish religious community." Was subject to further unclarity, among others, not looking at any timeframes or reversal in affiliations, besides the obvious contradiction to opt for religious status out of sheer & brutal civil servant practicality: the law makers argued at the time that this would make life easier for the administration.