As I said, the mined fee has a lower, not higher fee. So this is very strange to me.
This is not strange.
RBF is, in simple terms, a second transaction that you will send with a higher fee to get a faster confirmation.
His first transactions with lower fee was not confirmed, so he sent a RBF.
But both of those transactions were confirmed at the same height, in different chains.
Miner A mined the RBF and miner B mined the lowest one (maybe the RBF tx was not broadcasted to him yet when he mined) at THE SAME TIME! It is a coincidence, and the longest chain will survive.
Both chains are valid now.
In the end the chain mined by miner B was the longest chain. Miner A lost his block reward, his chain is not valid anymore and all transcitons in his block are not valid (but most of them were mined by miner B at the valid chain).
So the RBF is not valid anymore, because you cannot doouble spend an already spent coin. there is no RBF of that transaction in the longest blockchain now.