Default Active Directory limit request result entries to 1000 entries. So when you have more than 1000 users in your Active Directory database, the "select group or user" popup window will fail retrieve the list of user and group.

It is not always possible to increase the limit for request because IT responsibility is sometimes far from people responsible for PlasticSCM server. Also changing the query limitation on Active Directory has an impact on the entire company infrastructure and it is a huge risk.

I have read Active Directory query can use "page search" to split request result (see link below). I do not know PlasticSCM arcane, but maybe paged search could solve the issue.
https://docs.microsoft.com/en-us/windows/win32/adsi/retrieving-large-results-sets