Sorry for the troubles. Thanks for sending the reproduction project and for posting here on the forum.
It seems as if the problem comes from the background skybox being rendered after the skeleton which is rendered in the queue AlphaTest (Transparent Cutout). Rendering the rather costly skybox at the end of the opaque queue (even after AlphaTest which counts as having no semi-transparent pixels) is quite common and a reasonable "optimization".
The issue should be resolved automatically when any real (opaque) content is placed between the skeleton and the background skybox, or the Camera setting is changed crom skybox to solid color and updated (cleared) every frame.
Another solution would be to set the shader Material property Render Queue Offset to 550 to change the skeleton from AlphaTest to Transparent.
