Re: CSS mystery…
Is this in a child theme from bp-default?
Have you checked the file path is correct to your custom.css file? Do an inspect element in your browser and check the path bloginfo is generating – is it the one you expect?
Also, not sure what you mean by it “it defaults to bp-default/_inc/css/default” … is this a child theme, or have you just tried modifying style.css in the bp-default theme itself?
If you have made a child theme, and followed the standard bp css structure – then style.css will include the default.css in your child theme’s _inc/css folder – not to the default.css in the bp-default theme.
If you clarify these questions, it might help explain which css files are actually being used.