Set clipsize: -1 for all your melee weapons and see if it solves the problem (it should, I have lots of combat experience with this). Not sure with GunMelee, this mod could be outdated thus causing some extra problems, but that's not very likely since it's an official mod. Note: aliens won't use GunMelee as far as I know, meaning they will use ranged weapons only for ranged attacks.
Curious thing with aliens using melee weapon they've been unable to after you show them how... But that's some unintended side effect, not a bug.