Thanks for reporting with the fully repro project! We're having a look at it and will get back to you as soon as we've figured out what's going wrong.
Thanks to your reproduction asset we could boil the problem down to the following code reproducing the problem as well:
state.setAnimation(0, "straight up", true);
TrackEntry entry = state.setAnimation(1, "straight up", false);
entry.mixDuration = 0.5f;
entry = state.addAnimation(1, "straight up", false, 0.5f);
entry.holdPrevious = true;
entry.mixDuration = 0.5f;
entry = state.addAnimation(1, "straight up", false, 0.01f);
entry.holdPrevious = true;
entry.mixDuration = 0.5f;
I just filed an issue ticket here:
[runtimes] Hold previous on 3+ animations causes dipping · #1738
We will let you know once we have come up with a bugfix.
A bugfix has just been committed, a new 3.8 spine-unity unitypackage is available for download here as usual:
Spine Unity Download
Thanks for reporting!