This problem did not occur in 4.27.0.
Animation is not output when outputting a hierarchical structure.
This is how the attachments are registered, and the key is placed in the Transform Track of each ABC.
Animation is output for both ABCs
AB's Animation is not output, only C's Animation is output
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-134900 in the post.