This is a little bit generic, I don't know what to do.
I don't want to break the shader, but it seems like I have to disable the straight alpha input in order to enable multi_compile in the Spine/Skeleton shader? Once I've done that there's no change in the application and no recompile so I don't know if changing it will actually flow on through to a newly compiled shader.
I believe I'm using the correct shader. Again, in dev my spine animations show and run on the device. But in production I think that this error kills it at runtime before it can do anything.
The runtime is up to date, seemingly (the github version is still at 4.1? No package manager prompt to update.)
Fallbacks: again, I'm not confident editing the shader.
Asset bundles and refs: yeah it looks like they're there, because the animation loads and plays.
Testing in a clean project: I am completely out of time and won't have a job tomorrow if I can't fix this, so there just isn't time to test it in this way. It would take me at least a week to set up.