Just a wild theory, and sorry for replying here and the other thread. Also sorry if you tried this.
BUT HERE GOES BECAUSE I ALSO HATE THIS BUG:
Do you by any chance have any charms / gear that gives you a tiny bit of Dex, so that with mostly hardpoints (and like a torch/charm with stat), it just allows you to wield the PB?
From my own issue, here’s what I think could happen in your case with Djinn Slayer.
Version A: I have naked stats to equip 1 of 2 weapons, but weapon 2 allows me to equip weapon 1. In this case, the wrong weapon will main hand (Ali Baba example).
Version B: I do not have naked stats to equip neither of the 2 weapons, as I am using a Torch to give me all stats, in which case no weapon is FORCED into the main-hand and therefore the main-hand stays the main-hand (Djinn Slayer example).
Again, this is only what’s been true for me. So I’m sorry if this is not true to you. But I managed to replicate what you were describing with the above logic.
Thread in question: