Environment is CRM 2016 SP1 on premise.
When a (case) record is [Active - Read only] a form custom relationship filtered lookup is still active even though the record cannot be saved.
Furthermore, assessing the form type in order to selectively hide the lookup returns a value of 2 (updatable) even when the record is readonly.
The only workaround is to use RetrievePrincipalAccess to check.
Thanks