The bug was introduced in #126291, which was a refactor PR. Prior to the
PR there was a condition that could only trigger when a given pointer
was null. That pointer was turned into a reference in the refactor,
but instead of deleting the conditional code that should no longer be
reachable, the refactor simply removed the null check, allowing the code
to still trigger based on just the remaining part of the condition.
The code behind the condition bailed out before checking all fcurves for
whether they should be included in pose slide, and thus would omit some
channels from the breakdowner (and presumably other operators as well).
This commit fixes the issue by simple removing that code entirely, since
it's no longer relevant. It also renames the function and one of its
parameters to be clearer about what it actuallys does, since that was
pretty obscure before.
Pull Request: https://projects.blender.org/blender/blender/pulls/127565
54584ea98d