Our team found that it was not so convenient to have avatars for assigned developers only for Plan mode and not for Work (Rapid board of Jira Agile).
I’d create a question about that issue on Atlassian Answers, but unfortunately the answer was “Unfortunately this is not possible due to the way User Stories are displayed in the system.”
I had a thought “why not to tweak it with userscript?” and solution came after:
You are welcome with criticism and suggestions.