It seems that adding a copyright custom parameter to an instance in a Glyphs project doesn’t override the copyright field from the source file in the exported font.
Overriding with a Name Table Entry parameter for ID1 does work though.
I’m on 1312, and wondering if this has been introduced recently because I thought this was working before.
I know I’ve had issues, at least at some point, with some custom parameters that were not working properly for which I had to use Name Table Entry instead (Name ID3 - Unique Id is one of them, not sure if that is fixed is more recent versions of Glyphs).