Microsoft Lists: First column doesn't work when duplicating a list

Looking at 2022 I'm trying to duplicate my 2021 marketing plan. All settings are duplicated as expected BUT there's an issue with the first / main column of my list: It does not appear in the window used to create or adjust list entries.

I checked all the settings available and it seems to be all right. The column is even set to require an entry for every list item, yet I still can add new items without it being filled... Did anyone have this issue before and knows how to solve it?