When I go to https://beehaw.org/c/music@lemmy.ml I see "0 subscribers". It appears nobody on Beehaw subscribes to this external community. This means posts and comments are not actively being pushed from Lemmy.ml to Beehaw. Until at least one Beehaw user subscribes to this community it will remain in a sort of limbo-state with missing posts and comments.
I have asked the Lemmy developers to implement more proactive "fetch posts/comments from remote" behavior for instances, but while they acknowledge this could be more intuitive it's not a priority on their roadmap.
To be clear this is not a bug as such, just how Lemmy federation works currently.