In Pro 2.1.2, add a layer to a Map, and select "Design Fields".
Add another layer to a new Map, and select "Design Fields".
So you now have two maps, and are "Designing" fields in two different layers, a perfectly reasonable and valid work flow.
Close the second map. Note how the Design Fields UI for that second layer in that second map is still open. It "should" close.
Delete the second map. Note how the Design Fields UI for that second layer in that second map is still open. It "should" close. The layer is no longer part of the project.