Before patch 3.1.0 key binding “SELECT WEAPON 3” was for secondary weapon.
So if you can’t select secondary weapon, just remap your key bindings. They moved primary and secondary weapon one position up…
SELECT WEAPON 1 = Plasma caster (shoulder cannon)
SELECT WEAPON 2 = Wrist blades
SELECT WEAPON 3 = Primary weapon
SELECT WEAPON 4 = Secondary weapon
Probably not a bug, just fix how the weapon slots were supposed to work since begining?