Assuming this is a bug, from the discussion of Changing ownership of posts:
Change ownership of a topic
Original author avatar still appears after changing post ownership - part 2
It’s connected to the longstanding flakiness of the topic ‘featured users’ code.
@riking Have you heard any updates on this issue or plans to fix in a future release?
It’s pretty low priority, as it’s only visual.
Is there any update on this? Just noticed this behaviour today and was about to report it myself.
Our plan is to get rid of the concept of “OP” that can be different to topic creator. This nuance has only caused us pain. As a side effect it should fix this.
I thought I fixed this, is it still an issue?
I’m pretty sure it’s fixed for a while.