As the question shows I have checked everything I can thing of
- Chatter is enabled
- Chatter Feed Tracking is on for the objects
- User object is Public Read Only for External Users
- Even set up a sharing rule to share users with All Partners
- Made sure users are members of the community
When in the community as a partner user, and I do @ - the only name that can be found is their own name. Note: I am in the community as Log into Experience as the contact record, but I don't think this would make a difference.
Any other settings I am missing?