Currently the user search REST API will limit responses to 100 users if a source other than "default" is used. This is done to prevent the API from causing out of memory errors when very large external authentications systems are used:
This makes some sense for external systems like LDAP and Crowd. It does not make sense for the "allConfigured" source:
Users from that source have been explicitly granted roles, and it should be possible to retrieve all of them.