Alternate CSS not working properly

I'm getting unexpected results when using the Design Manager (publishing features) to refer to an Alternate CSS URL which is located in a seperate site collection (<tenant>.sharepoint.com/sites/assets/Style Library/custom.css).  For me, I appear to see the custom CSS as follows:

but for some reason other users don't appear to be getting the customised CSS - I have checked the permissions on the assets site and library and all users should have read permission, they see this:

I'm a bit stumped as to why this is happening? Any ideas?

cheers!