Well, I’ve had a fiddle with my SX2 in 4 deck mode and you’re right, I only use two decks so never noticed.
Anyway, the issue is, that in the mapping table CC2, CC4, and CC6 are not available for decks 3 & 4.
With those not available to map it defaults to decks 1 & 2, regardless of what deck you have selected on the controller.
I’ve had a play with the mappings and found a workaround, but it’s not very neat….
You can duplicate the mappings on CC2,4 &6 for decks 1 & 2, and then reassign the duplicates to decks 3 & 4.
However, the big caveat here is that while it will control the FX on the allocated deck, it will also control the FX on the non selected deck, so if you changed the deck 2 FX, it will also change the deck 4 FX simultaneously!!!
Not ideal, especially if you’re on. It’s decks 2 & 4 at the same time….
The only solution I can see would be to put a bug report in to support and ask them to add CC2, CC4, and CC6 into the mappings for decks 3 & 4.
Although the fact that duplicating the control ends up in both decks activating together might indicate it’s a limitation of the hardware……
Decks 3 & 4 might need additional CC controls adding to their respective mappings.
Not great news, I know, but at least you know why the behaviour is happening.