Thanks LondonAussie, I went into incognito and choose the room and didn't log in until the very end, and then got a technical error. It seems that I can only book rooms with paid executive lounge access. I guess this means they do not want to grant the lounge access to Accor Plus members at that Pullman. Luckily there is another very lovely place on the same island, so I'll just book that instead.

This is the first time I've come across the higher rate while logged in with Accor Plus, although I've read other forum convos on that. It's a bit disappointing, and it means I'll always want to double check that I am actually getting a decent rate, which is also annoying to have to do that bit of extra research. But I can live wirh it.