I'm not sure if it's a bug, maybe it should work like this by design, but it would be good if someone clarify it, here is what I do:
1) Create a Transform Constraint
2) Set it to be Local and Relative
3) Set a Rotate Mix to 100%
Result: Rotation Mix works fine, but from now on constrained bone's local Scale doesn't influence a bone. I tested it in Spine 4.0.26 and in Spine Unity Runtime 4.0-2021-08-16
My conjecture: I checked a Spine Unity Runtime Github, here is a part of ApplyRelativeLocal method of TransformConstraint class from Spine 3.7 Runtime [https://github.com/EsotericSoftware/spine-runtimes/blob/3.7/spine-csharp/src/TransformConstraint.cs] :
...
float scaleX = bone.ascaleX, scaleY = bone.ascaleY;
if (scaleMix != 0) {
scaleX *= ((target.ascaleX - 1 + data.offsetScaleX) * scaleMix) + 1;
scaleY *= ((target.ascaleY - 1 + data.offsetScaleY) * scaleMix) + 1;
}
...
And the corresponding part of the code from Spine 4.0 Runtime [https://github.com/EsotericSoftware/spine-runtimes/blob/4.0/spine-csharp/src/TransformConstraint.cs]
...
float scaleX = (bone.ascaleX * ((target.ascaleX - 1 + data.offsetScaleX) * mixScaleX) + 1);
float scaleY = (bone.ascaleY * ((target.ascaleY - 1 + data.offsetScaleY) * mixScaleY) + 1);
...
So my guess is that there could be a typo and it should be something like this instead:
...
float scaleX = bone.ascaleX * (((target.ascaleX - 1 + data.offsetScaleX) * mixScaleX) + 1);
float scaleY = bone.ascaleY * (((target.ascaleY - 1 + data.offsetScaleY) * mixScaleY) + 1);
...
