@Gabriel_Wink Please also check whether you've disabled Animation clean up during skeleton export. Otherwise keys identical to the setup pose will be removed, unless Layered is enabled at the animation.

@Gabriel_Wink Thanks for reporting and sorry for the troubles. While this was intentional to make the outline-only case smooth on the inside, it indeed causes a slighly transparent gap as you've described. We've fixed the issue by providing a new property Fill at all outline shaders. It can be enabled to fill the transparent gap and defaults to disabled to keep the existing behaviour.

This issue has been tracked under this ticket:
EsotericSoftware/spine-runtimes2758

From the changelog:

All Spine Outline shaders, including the URP outline shaders, now provide an additional parameter Fill. Enable it to also fill the opaque area inside the outline with the outline color. Prevents a semi-transparent gap between outline and skeleton. Defaults to disabled to maintain existing behaviour.

New spine-unity and Spine UPM Shaders URP 4.2 packages are available for download:
https://jp.esotericsoftware.com/spine-unity-download

Please let us know whether this resolves you issue as well.