[animation-mixer] timeScale -1 not working for gltf animation

[udl]

Hi there and thank you for your great work!

I tried to trigger a gltf animation and then reverse it. Triggering the animation works. But the backward direction doesn't work. It seems, several people had this issue in the past (https://github.com/c-frame/aframe-extras/issues/364). I've created a glitch demonstrating the problem: https://glitch.com/edit/#!/thoracic-typhoon-rest You can click "forward" and it works just fine, "backward" doesn't work unfortunately. Or am I doing something wrong?

Thanks in advance and let me know if anything is missing to reproduce the issue.