I have always used the mouse wheel for weapon change. suddenly after summer squash I got a bug that when I scrolled down, I switched to primary weapon but then a fracture of a second later to secondary. Mind I had bound toggle prev/next to mouse wheel. I could fix this by binding primary weapon to mouse down and secondary to mouse up. This has worked for weeks and now suddenly the bug is back.
I would like to ask you. Could you just bind primary to wheel down and secondary to wheel up and scroll really fast/strong/hard down and see if you get the weapon you wanted. You have to do this from holding your melee. If you already holding primary/secondary everything is fine.
I would like to know, if there are others who have this problem, because it is driving me nuts. especially after I thought, I found a workaround.