I am FB Platinum and made the mistake of flying CO extensively last year.
Lounge access; be it CO or partner; is and should be granted regardless of cabin (if on an intl ticket if ex-USA) when flying CO.
I was never denied access when Platinum, was denied access once when Gold and got 2500 EQM for my trouble(s).
Write a nicey or nasty letter to FB explaining the situation.
UTA