I'm looking at booking a CX flight using either avios of aa miles and was surprised to see no availability on ba.com vs availability (up to payment page, i haven't checked out yet) on aa.com

I know some airlines make more seats available for their own members but isn't awarded availability for partners (for booking with miles by their loyalty programme members) meant to be the same for all partners? If so, is this a known glitch and/or common issue? Asking before I potentially query it with BA's ggl line.