After upgrading to a newer Clarity release, a 'None' value appears inside lookups that do not require a value in the New UX.
In earlier releases, it was possible to nullify a pre-existing value in a lookup that does not require a value by deleting its value.
As part of the refactoring on lookups, there is now a 'None' value so you can nullify it.
This is working as designed. If the attribute is custom and should always have a value, make this attribute required.