1 Add a post-function "Comment issue Function" to a transition
2 User this transition with your user ( Username: pmueller, name: Paul Mueller ). Works; user will be mapped correctly as comment author. (green in screenshot)
3 Rename your user's username from pmueller to pmeier
4 Use transition again
5 Comment author will not be mapped to your user correctly, but added as plain text "pmeier" (BUG). Hovering the user works, but name and avatar are now shown. (red in screenshot)
I have seen similar problems in other JIRA plugins since Atlassian allowed renaming of user names.