Are you able to @ mention users in the Description field?
I did a Google search for "Jira cloud mentions not working" and found several posts on the topic.
A KB article from Atlassian: https://confluence.atlassian.com/jirakb/mentions-and-autocomplete-are-not-working-in-jira-cloud-779160727.html
The first diagnostic step is to check that you have the "Browser Users" permission which is a global permission. You did mention that you had already checked permissions. Did that one get checked and was it confirmed that you are a member of a group that has been granted that permission.
Catch up with Atlassian Product Managers in our 2020 Demo Den round-up! From Advanced Roadmaps to Code in Jira to Next-Gen Workflows, check out the videos below to help up-level your work in the new ...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events