This has also been happening to me since I picked up Eccea. The animation loop stops if I equip a melee weapon in the main hand slot and it functions normally when there is a pistol or blunderbuss in the off-hand. Unfortunately, equipping any firearm in the main-hand (both one-handed and two-handed) restarts the endless looping. Additionally, I'm not sure if these issues are related, but the Conductive Blast ability on the Thundercrack Pistol stopped refreshing upon rest when this reload issue began happening.
Edit: Equipping a melee weapon in the main hand seems to be a temporary fix and no longer works. The Watcher restarts the loop after firing a single shot. The reload loop persists throughout combat and doesn't reset on a save file load or full game reboot.
