moti However, animation B already has the same key (of the constraint Rotate Mix value) as frame 54 at frame 0 in the project I sent you.
The exported SpineTransformConstraintToUnity.json
skeleton file does not contain the key at animation B
on frame 0. If you placed one there, did you perhaps export with Animation clean up
enabled? If so, please disable this setting when using multiple tracks, as it it removes keys identical to the setup pose.
If you mean that the animation B
looks and behaves as if there was a key on frame 0 in the Spine Editor: this is because the setup pose (with value 0.3, or 30%) is used as a base value before your animation is applied on top, and your animation later on frame 54 sets the same value (also value 0.3) as the setup pose. Therefore the problem just surfaces when the animation is layered on top of an animation which does not set the value to 0.3.
If I misunderstood your question, could you please explain it in more detail or in different words?