Allow adding N:N relationship from inactive case

Dynamics 365 online v9.1

I have created a "Tag" entity and a N:N relationship with case. This allows us to add multiple tags to a case to make searching easier.

This is fine while when the case is open but I can't do this from the case when it is resolved. How do enable the '+' on the subgrid on the case form as it is with the Child Cases subgrid.