Today I wanted to update my old project (Starling-AS3). But with updated runtimes all animations were screwed. SetEmptyAnimation() with mixDuration don't mix out a track to SetupPose anymore.
Edit: this bug appears in "spine-runtimes-3.6.32" release. Version 3.5.51 is working fine.

If we open Spineboy-Example and change animations to these, it works ok:

skeleton.skeleton.setToSetupPose();
skeleton.state.setAnimationByName(0, "run", true);
skeleton.state.addEmptyAnimation(0, 0, 3);

But if we add mixDuration, then the boy does not return to its setup pose and just freezes:

skeleton.skeleton.setToSetupPose();
skeleton.state.setAnimationByName(0, "run", true);
skeleton.state.addEmptyAnimation(0, 1, 3);

Also changing one animation to another with addAnimationByName() does not reset slot visibility keyframes from the first animation to SetupPose:

skeleton.state.setAnimationByName(0, "idle", true);
skeleton.state.addAnimationByName(0, "action", true, 3);

Thanks, badlogic!