Personally I just fixed bugs that rendered the game unplayable but didn’t introduce new features so it’s still fair and doesn’t give me an advantage over others
I.e. my games FPS was really bad, like 20fps at times b/c of high vert count, so I fixed it and submitted, but I’m not adding full features like proper health bars and new content
Basically it’s allowed but up to your judgement, just don’t go too crazy with the updates - simple bug fixes should be fine from my understanding! 🙂