itch-side, yes. Your jam entry is just a link to your project, so you can push a new build any time, deadline or not, and the entry stays valid. Judges launch whatever build is live when they play it, so a fix does land.
The jam rule here is about how the game got made (inside the window), not about freezing it afterwards. A bug fix is a fix, not a new game, so it should be in bounds, but that call is the host's, not itch's. slapjam/tobias are both active on this board if you want it confirmed.
FWIW I hit the same thing at the deadline, pushed a fix, and the new build was served straight away.