Cannot see all users when adding users to a group

[Annie M1]

I'm a contractor dev for a client where there are multiple projects happening at once including custom scoped app creation, HRSD implementation, and a SailPoint integration that relates to user fields. Something happened in our dev instance a week ago that we cannot pinpoint or figure out where to look. What is happening is this: 

1. Open any group record (sys_user_group)
2. Click Group members related list > click Edit UI Action 
   • Expected outcome: slushbucket on the left shows all users 
   • Actual outcome: slushbucket shows a very limited amount of random users, like about 50 or so, and we can't tell how they're related, some are active, some are inactive, some are direct client employees, some are contractors

Have asked other devs in the instance if they did any recent changes related to filtered lists of users and no one thinks so. I just don't know where to see what happened or debug this. I'm suspecting the SailPoint but can't find what it would be. Where do I start? How does one even filter the user collection on the group member list? 

[Annie M1]

One more thing I found interesting. On the Edit Group Members page, when I create a filter such as First name starts with Michelle, I get a LOT of Michelles, most of which didn't show in that initial limited list. Same thing if I search on a last name. So, filtering seems to "awaken" the list of users I can choose from, but they aren't all showing at once on their own like they used to. Hope this helps with ideas for where I can look?