Hello,
I'm on PC with Spine 3.5.51.
If I'm removing my image from the folder 'images', of course I have a 'missing' warning.
So when I'm doing the export process, I obviously have the error message.
But, if I put back my image in the folder, and let's say by mistake I renamed my slot, and 'Rename attachment' was also checked, then, my export is correct. Yet I have a missing image.
I spent a lot of time finding what was wrong, because the missing asset was hidden on the setup.
If you think you can do something, it'd be cool.