poiipoii wrotein the first frame of animation, when a bone is not keyed, Spine uses its setup position to begin with.
Yep. Later versions of Spine show it using a dotted line, just like a stepped key:
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Note if you are playing multiple animations at once (see Preview) that not keying something may result in the pose from an animation on a lower track rather than the setup pose. Only if no lower track keys it, then you get the setup pose.
poiipoii wroteis there a way to reset the setup bones' positions by their first frame animation positions
You can copy/paste bone transforms, so you can copy a pose from an animation to the setup pose. However, if you change the setup pose, all animations are affected.