The issue you're experiencing with mixing animations when deform keys are present is likely related to the "dipping" problem. This occurs when animations with keyed values mix, causing the values to dip toward the setup pose before returning to the intended keyed values. This behavior is a result of how mixing is handled in Spine, where the transition between animations can cause unexpected movements if both animations key the same properties. The presence of deform keys can exacerbate this issue, as they add complexity to the mixing process. Unfortunately, this is a known challenge with mixing animations that involve deform keys, and while there are some workarounds, such as adjusting the mixing settings or using weighted meshes, it can be difficult to completely eliminate the problem.

The fix is to use the latest version!