Thanks for reporting, we could reproduce this issue.
Unfortuantely this seems to be a current incompatibility of the 2D shadow casting system with MeshRenderer in combination with Texture transparency. The same issue also occurs when creating a normal Cube with a Universal Render Pipeline/2D/Sprite-Lit-Default material and assigning any Texture with transparency in it. The same Texture used as a Sprite at a SpriteRenderer works as intended, as you can see below:
Unfortunately none of the Texture Importer or MeshRenderer Inspector settings changed anything in this regard, so we're running out of ideas on how to fix this. It seems as if it's currently only working with SpriteRenderer but not with a MeshRenderer.
A very poor workaround would be to enable Self Shadows at the ShadowCaster2D component, and countering the missing light in the shader. This is a very limited workaround however, for obvious reasons.
Perhaps some thirdparty 2D shadowing asset packages might work instead of using Unity's 2D shadow casting system. If anyone has any recommendations in general regarding 2D shadow casting (volumetric lighting) in combination with Spine Skeletons, any input would be much appreciated!
