It is 1350 when Ottoman had conquered Constantinople and made it new capital. In next turn, Ottoman will conquer Naissus in Balkans and their capital will automatically move to Ankara again.
I assume it's because all national wonders will be destroyed upon city capture. When capturing Constantinople, Ottoman will immediately be granted a palace in it (relocateCapital in RFCUtils.py). In next turn, Constantinople is still in anarchy so the new palace is destroyed. Since there exists no palace at all, the system will automatically assign a new palace for Ottomans, which is the first-ID-city Ankara.