Can't put an animated sprite on top of the hero

Frozen at frame 0, thought bubble doesn't move.

  -- Rachel has a thought bubble animation
  function hero:think()
    self:freeze()
    local thought_bubble = hero:create_sprite("menus/thinking", "thought_bubble")
    function thought_bubble:on_animation_finished()
      hero:remove_sprite(self)
      hero:unfreeze()
    end
  end

I have an animated sprite I want to put on top of the hero. If I call hero:create_sprite("my_sprite") the sprite appears, but it's stuck on frame 0. Nothing I do makes it move.

If I apply the exact same sprite to an NPC, it works as expected. This leads me to believe the hero specifically has issues dealing with sprites added at runtime.

The hero has some strict requirements on its animation sets. This may be related to #1235.

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