Alright, I have a PR up that clears things up.
Key takeaways
We now pass the username when
siteSettings.display_name_on_posts && !siteSettings.prioritize_username_in_ux && post.name
as well as the full name to guarantee that we are not getting any mismatches when querying for user / avatar.
eg.
[quote="Isaac Janzen, post:3, topic:7, full:true, username:isaac.janzen"]
bing bong
[/quote]
What are your thoughts @sam?