CSS File assets are just static files, so they won't do any matrix parsing of the content before being served, so none of those methods would be re-written to full urls.
./?a=1234 might still work even though it doesn't get parsed.
If you want to use those references, I suggest using a CSS Design Asset instead, but can't remember if both methods work or just ,/?a=
Any reason why you are not using CSS Design Assets instead?
Thanks Bart, Cheers for the info. I thought as much but was puzzled why these worked in the past (just luck I guess).
I'll look at updating these to CSS Design Assets. I can't recall why these weren't previously created as such - I vaguely recall some issues around performance with CSS Design Assets at some stage which may have spooked us.