Pressing MMB whilst aiming down sights is just melee.

Video evidence:
https://streamable.com/5ne16l

I just tried it, working for me. Did you double-check your keybinds? Sometimes the CTE messes with them. I used the "Melee Attack/Cycle Optic" option on MMB, I have the separate options for Melee Attack and Cycle Optic unbound.

@Peglegswansoon_ Ok yeah MMB was bound to both cycle optics/melee and melee, now it works when I removed the melee only binding, thanks