Semaphore's hypothesis was right.
I found interesting resource which tells us that islands actually had been considered terra nullius till 1926.
Until the year 1926 the islands had been considered "Terra Nullius", or other words, "No Man's Land". However,
following practices of Canada, the Soviet Union claimed that all land
in the sector between the Soviet mainland and the North Pole was
Soviet territory. This met with criticism from the Norwegians who
refused to recognize the islands as being a part of Russia. but they
were able to do little with their complaints against the much larger
Soviet authority. (1)
However, that decision had little practical impact immediately, and even on the official Soviet maps issued in 1926,1928 and even 1929 (image below) Franz Josef Land was marked as being outside of the Soviet Union. (2)
Norwegian government officially protested in Moscow against this unilateral decree of annexation. Also, fascist government claimed sovereignty over the archipelago in 1928 after the disaster of the Nobile expedition - arguing that the Tegetthoff (named after Wilhelm von Tegetthoff) was equipped with an engine from the now-Italian city of Trieste, but non of these protests had any success.
(1) Barr et al. 1995
(2) Spitzbergen with Frank Josef Land & Jan Mayen (Bradt Svalbard, 201)