For my preferred date of travel, I'm waitlisted MRU-LHR in A. BA MRU say there is no availability in F or A. However, ExpertFlyer shows F4 A2 and a wide open seatmap.

If I try to book MRU-LHR on ba.com, I get an error message. However, if I book LHR-MRU return (with MRU-LHR on my preferred date) they are happy to sell me a seat, at the A fare.

Any ideas what is going on?