Hey,
I've encountered this behaviour when previewing a simple animation I'm working on.
This trapdoor just moves to the right (x) and has a bit of anticipation via animation curve, like this:
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The weird behaviour is that when I preview the animation, this little anticipation only appears when I play it looped AND only from the second loop forward. See the video:
It's not really a big issue, but I just wanted to share it in case I'm doing something wrong or this happens to be a bug.
Cheers!