When moving slots (with all their associated animation data) from one
Action to another, tag the data-blocks that are affected by the slot
move, so that they get re-evaluated. Without this, their (out of date)
evaluated copy would still point to the original Action, which no longer
has animation data for the moved slot.
Pull Request: https://projects.blender.org/blender/blender/pulls/136454
6ffb0e368d