As far as I can tell this bug only happens with breech loaded grenade launchers (both wave frames and lightweights) and not any other weapon that only has 1 shot in the mag. If you wait and let the game automatically start and finish your reload animation when your GL's tube is empty, your reload button will no longer work and you'll be stuck using slower automatic reloads until you stow and reequip the GL or until the bug mysteriously fixes itself. This bug consistently happens after a full, automatic reload and it seems to be a bit worse on remotely detonated grenades, but sometimes the problem fixes itself if you mash reload and fire a couple more shots. You can also circumvent the bug by pressing reload while sprinting (works with toggle sprint, not sure about hold to sprint.)