Playing backwards discrete animation will begin with the first keyframe

[KoBeWi]

Tested versions

4.3 beta1
not sure when it started exactly

System information

Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce GTX 1060 (NVIDIA; 31.0.15.4633) - Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz (8 Threads)

Issue description

I have a door that uses the same animation for opening and closing, and the animation is played forward and backward.
It looks more or less like this:

Notice the empty space between last frame and end of the animation. When playing backwards, the empty space is filled with the first frame:

godot.windows.editor.dev.x86_64_lBjNvI3kuE.mp4

It works correctly when I repeat the last frame at the end:

MZf0Ad0fdD.mp4

This wasn't necessary before.

Steps to reproduce

Run the MRP.

Minimal reproduction project (MRP)

discretebug.zip

[JekSun97]

This happens starting from version 4.3 dev4, which I also noticed, this only happens in a running project, I don’t see this problem in the editor