New comment by rdslw on void-packages repository

https://github.com/void-linux/void-packages/issues/47011#issuecomment-1890431965

Comment:
Thanks for answering it! 

The actual reason is to have ability to writeable (some) files on /boot vfat partition, while not allowing all users to write them. Here gid points to some wheels/adm gid which only some users belong. Due to the limitations of vfat fs, it's widely adopted to use some form of gid/uid/fmask/dmask on such fs, to have simple access control.

(Just to make it clear: xbps is being run under root) 

But moving back to the error:
I think "xbps should ignore failures" is not true, as there is no ownership on vfat filesystem type (used on /boot) therefore this is not a failure, but xbps bug (not understanding that sometimes this operation (permissions/ownership) is not needed)

I consider FS settings machine, not void specific. One can ask, why using vfat on this filesystem? ;)