I have 1 user in my team that used to be able to @ people when she first joined, but suddenly lost the feature, and I can't figure out why. She has all the same permissions as everyone else in the team, I've tried making her a trusted user, I've even tried deleting her entirely and readding her and nothing is making a difference. When I switch to her account I also have the same issue, but on my own account it's fine.
Anyone have any suggestions as to why this might be happening?
Please have a look at this article: https://confluence.atlassian.com/jirakb/mentions-and-autocomplete-are-not-working-in-jira-cloud-779160727.html
Most likely is that the user doesn't have the Global Permission to 'Browse Users' and as such no users would be returned after typing '@'. I know you mentioned this did work originally, perhaps there was a change made here.
Hope that helps.