@notme1611 The lines in the error messages neither match the code of the 4.1 nor the 4.2-beta repository, line 99 for example is an empty line on both branches:
EsotericSoftware/spine-runtimesblob/4.1/spine-unity/Assets/Spine%20Examples/Scripts/Sample%20Components/RenderExistingMesh.cs#L99
EsotericSoftware/spine-runtimesblob/4.2-beta/spine-unity/Assets/Spine%20Examples/Scripts/Sample%20Components/RenderExistingMesh.cs#L99
Did you modify the RenderExistingMesh source code? If so, please share what code now lies on the mentioned lines of the error messages (lines 99, 133 and 150).

@notme1611 Thanks for sending the reproduction project. The problem with your mask shader Spine-Skeleton-Mask.shader that in line 45 it incorrectly reads float2 _Cutoff; (a 2D vector) instead of float _Cutoff; which makes Unity not assign your configured Alpha Cutoff Inspector Material values at this shader property, and assigned 0 every time instead.

BTW: Since I saw you were using two identical SkeletonAnimation GameObjects for hero_2_visual and hero_2_visual_silhouette: Please note that you could use the RenderExistingMesh component to re-render an identical skeleton mesh with a different shader. While it might not be included in spine-unity 3.8, you should be able to just integrate the example component script taken from 4.1 into your project. You can find the source code here. If your setup was only for the reproduction project, please ignore the section above.

@notme1611 Sorry to hear that the issue persists. I can see nothing obviously wrong with your shader though.

At the SkeletonAnimation component, did you set up the Mask Interaction property the same (e.g. to Visible Outside Mask) as with your non-Spine Sprite?

If it is set up the same, could you please send us a minimal Unity project which still shows this issue? You can send it as a zip package to contact@esotericsoftware.com, briefly mentioning this forum thread so that we know the context. Then we can have a look at whats going wrong.