SharePoint @mentions limited to random users

Hi everyone,

We are hitting a strange issue with SharePoint @mentions where an affected user can only tag themselves and a few random employees, instead of the actual members of the site. I’ve attached a screenshot of what they see when trying to tag colleagues.

The Details:

• The Glitch: Typing @ only populates a tiny fraction of the directory (see screenshot). The affected user (Michael Drabek) can tag me and himself, but none of the other 14 active members of this 16-member SharePoint site.
• Cross-Testing: Another user on that exact same shared site is able to tag Michael successfully, so his profile is discoverable to others—he just can't see them.
• Environment: This happens identically on both the web browser and desktop apps.
• Verified: Michael has the correct SharePoint site permissions and full Microsoft 365 licensing.

It feels like a hidden Entra or User Profile Service sync glitch rather than a standard permissions issue, as his directory lookup seems completely restricted.

Has anyone seen the person-picker index get broken like this for a single user? Any tips would be appreciated.

Thanks!