Just to clarify — the bug isn’t about these options being mutually exclusive.
The real issue is: originally, a field can have both “Editable after signup” and “Show on signup” unchecked. But once you check “Editable after signup” even once, the frontend enforces that at least one of these two options must be selected from then on.
Fields that are optional but not editable need to be on the signup form, or they can never be filled in
The goal of the current implementation was to prevent admins from creating a user field that is neither editable nor shown on sign up. So if something is a bug, then it’s that you can still create the field without these properties. I would prefer if instead of fixing that, the @team would consider the use case of staff-only user fields and fulfill that feature request. That’s why I posted in the feature topic.