Ok, thanks. I just wanted to check if it was something in my setup causing it, but seems like a global issue.
Right Alt + [ = on/off for left main MFD -AND- next page on small left aux MFD.
Right Alt + ] = on/off for right main MFD -AND- next page on small right aux MFD.
I think this "doubled" behaviour is because Right ALT is the same as CTRL, and CTRL [ ] switches the MFDs on/off. But it's also behaving as ALT so changing the small MFDs as well.
I think to be safe, it's best to use Left ALT all the time to get the expected behaviour.