It actually isn't Flarum's fault (but it can be fixed by Flarum!).
It's because the BA embedded in the original post is technically part of a list of things to go in the post footer and browsers tend to display lists in lists (in this case the list inside the post footer which is also a list) differently.
https://jsfiddle.net/1Ln52ba0/
Bullet points in posts are solid. Bullet points in BA are hollow.
