Override Animation Data doesn't use the new animation to play when the animation mode is already 'Use Animation Asset'
Create a BP from the mannequin and place an instance in level.
In the BP on an input event use a flip flop to call Override Animation Data with different animations set and IsPlaying as true.
PIE and trigger input event multiple times.
Notice: Only the first animation set, including via the instance/editor will be used when playing.
Expected: The playing animation would change to the new AnimToPlay.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-84805 in the post.