The issue you're experiencing with black blocks appearing during animation when using the URP Lit shader and enabling receive shadows might be related to shadow casting settings or shader configurations. It's recommended to check the shadow bias settings on your light source, as incorrect settings can cause shadow artifacts like shadow acne. Additionally, ensure that your shader settings are correctly configured and that no modifications have been made to the Spine URP shader files. If the problem persists, consider testing with a non-Spine shader to determine if the issue is specific to Spine or related to the Unity setup.

@bigasa It's very hard to diagnose just based on two small screenshot regions without knowing your scene. A quick guess is that you are perhaps having an almost fully transparent attachment (e.g. alpha 0.05) enabled in the problematic animation which is casting shadows (in conjunction with a low shadow-casting alpha threshold value).

If that's not the case, could you please send us a minimal Unity project that still shows this issue? You can send it as a zip file to contact@esotericsoftware.com, briefly mentioning this forum thread URL so that we know the context. Then we can have a look at what's going wrong.

@bigasa Thanks for the additional info. Another potential cause might be if you've set your main or secondary light settings in the URP Render Pipeline Asset to Vertex Lit, this would explain different discontinuous lighting on attachments. E.g. when the vertices of a small RegionAttachment quad are evaluated per vertex and then interpolated, this might yield a completely different result than the vertices of a 3x larger attachment. You could have a quick check whether your URP Render Pipeline Asset has the lights configured that way.