@spudonkey It might be that some global setting is ignored.
Which Unity version are you using, and do you have URP with a 3D or a 2D renderer active?
How are you setting the fog?
Which exact version of the spine-unity URP Shaders UPM package are you using?
The easiest way to provide all settings would be if you could provide us with a minimal Unity reproduction project which still shows this issue. You can send it as a zip file to contact@esotericsoftware.com, briefly mentioning this forum thread so that we know the context.