I used to be able to undo literally a hundred steps in my scene using H12, but after ungrading to H12.5, I can only undo at most a few times but usually 1 or two steps, is it a known bug or there is something change in how undo works in H12.5?

Thanks in advanced.

Which OS are you on and also what type of operations are you doing? Also double-check in the preferences for the amount of memory that can be used for undos. (Or, that might be in Edit > Undo Manager)