Hey Simion,
Not a bug, I’m sorry to be the bearer of that news!
It’s actually a function - kind of a necessary one - of the fact that the first level of subtasks are displayed in the main project list view now, meaning that any custom fields that are showing as columns in that view are now available to those subtasks to be filled in.
Really the only other alternative to the behavior they chose to implement (i.e. allowing you to enter data into those columns, which necessitates adding those custom fields to those subtasks) would have been to block you from entering data into those columns for subtasks. But that would arguably be confusing and would feel inconsistent (i.e. “why can I enter data into columns for some rows in my project but not others??”).