I can't @ mention someone through a proof

Here are a couple links that might help. I think it depends on if you created the proof and the profile of the person you're trying to mention, if they're inside or outside of your company.

https://one.workfront.com/s/document-item?bundleId=the-new-workfront-experience&topicId=Content%2FReview_and_approve_work%2FProofing%2FReviewing_Proofs_within_Workfront%2FComment_on_a_proof%2Ftag-users-to-share-proof.htm&_LANG=enus

https://one.workfront.com/s/document-item?bundleId=the-new-workfront-experience&topicId=Content%2FReview_and_approve_work%2FProofing%2FTips_Tricks_and_Troubleshooting%2FCollaboration_with_Members_outside_of_your_organization.htm&_LANG=enus

I had a similar experience with a couple of people on my team. They kept trying to "@" a person named Kimberly, whose name always popped up when we typed "@kim" - however, that wasn't working that day. We discovered that typing "kimb" worked, and at least the name showed up and we could tag them. But we also discovered that typing "kimbe" did not work, so it was extremely bizarre. So maybe try to slowly type in the person's name, one letter at a time. We were never able to find out from WF what caused it. And it only happened that one day. Good luck!