Steps to replicate the issue (include links if applicable):

• Using discussion tools on enwiki click reply
• Click the mention a user button or type the @ sign

What happens?:
Only one name comes up in the pre-populated list

What should have happened instead?:
All editors who have participated in the discussion should show up

Software version (skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):
This isn't happening for me at all talk pages but yes to most of them. My user talk is one such talk page where it doesn't work. User:WhatamIdoing (WMF)'s talk does work. I am using Chrome 105.