The issue was that the function `animfilter_action` got a new
code segment at the start which checked `if (action.is_empty())`.
That function didn't check if the `groups` list of the action is empty
though. Regular keyframe animation is usually sorted into
keyframe groups, which means it is not stored under `curves` of the action.
However in the anim filtering code, the function `split_groups_action_temp`
moves the fcurves to their groups under the `groups` listbase.
Pull Request: https://projects.blender.org/blender/blender/pulls/124172
c446813dd3