the game will automatically switch to fullscreen window mode when an IME language is detected. it has been for years (possibly as long as the game exists ?!) so it’s not a bug but intended behaviour (or design choice ?!) - and no, i have no clue why…
however i haven’t looked into this issue for quite some time tbh so things may have changed (though i doubt it)
Why do you think it’s not a bug just because it exists for a long time?
It’s a bug that was introduced at some point (definitely not as long as WT exists, the issue was not present in 2016ish) and then either ignored or not acknowledged.
There is no reason to justify this behavior. Other games don’t have similar flaw, never seen any game that would refuse enter fullscreen with Japanese keyboard.