Directory Server and PAM passthrough is not working as expected

Issue

In our Directory Server environment we have:

suffix: dc=fooinc,dc=com
subtree: ou=people,dc=fooinc,dc=com
users ou: cn=bla,ou=people,dc=fooinc,dc=com

We can use these users correctly for the usual bind attempts like ldapsearch -D cn=bla,ou=people,dc=fooinc,dc=com -w password ...

Now we would like to have a second set of users for authenticated binds. But these users should have their passwords checked with Active Directory and not with Directory Server. For this we configured pass-through authentication as documented here:
https://access.redhat.com/site/documentation/en-US/Red_Hat_Directory_Server/9.0/html/Administration_Guide/pam-pta.html

To distinguish between authentication methods for different users we use the pamfilter system. We defined a second tree with other users to accomplish this: cn=aduser,ou=peopleAD,dc=fooinc,dc=com

The issue is that whatever we set pamfilter to all bind authentication attempts go straight to AD. We would like only the users within cn=aduser,ou=peopleAD,dc=fooinc,dc=com to be authenticated against Active Directory.