I posted about the AnimationPlayer shader parameter interpolation problem on t/suki and mentioned a few more details:

• The problem happens in Godot 4.2.1
• You have to set all key frames in all animations in order for this workaround to work. When I first tried doing this, I found that the interpolation for my animation still wouldn’t work because the RESET animation still had a key frame with null values.

See: forum.tsuki.games