The issue faced:
We got a string collection claim that holds some preset GUID returned from an Azure AD tenant.
We wanted to map this GUID into another values so that the GUID will get translated into a readable text. Currently there is a similar string claims transformation: LookupValue, but there is no similar claim transformation for string collection type.

Currently we can solve this issue by adding a helper Rest API endpoint to do this conversion, but it will be helpful that the custom policy natively supports this claims transformation.