TimVervaet Hi, I had the same issue and it’s really sad that no one from Microsoft has fixed nor replied to you!
I got out of my way to register this account. Way to complicated if you just wanna answer someone’s question, if you ask me 🙂
Anyways, yes it seems to be a simple bug!
When you scroll down on the bottom left you can actually still access the „classic sharepoint“ view. I guess it’s coded different but there you can also add column. I found my bugged one there, unhide it and re-enter the normal SharePoint view. My column was then visible, allowing me to either use it or delete it and creating a new one.
Worked for me and I hope the tipp to use the classic sharepoint works for you and others as well.
Cheers