Shout-out to Host_6 for teaching me that. But that's not stopping me from experimenting the causes of this bug.
Since this bug, I've been having conversations with people in the Nox Discord, and I've been exploring this bug more and more.
A few people brought up the fact it might have something to do with the "auto-equip shield" feature in Nox Reloaded, and now that I'm aware of this feature, without a shadow of a doubt, that's part of the issue. So here are the conditions:
You have to swap to nothing, or drop something whilst NOT having a secondary weapon. This affects Conjurers and Wizards if you swap to your fists. Warriors can be affected by this bug if they swap to nothing without a shield. Probably because the Warrior wants to switch to a shield he doesn't have. Likewise for the Conjurers and Wizards, because they want to switch to a shield that they can't equip because their class forbids it.
Yes, this is helpful for Warriors, but it's not helpful if he's swapping to a 1-handed weapon and he's NOT wearing certain armor if he does NOT have a shield, especially in quests. In PvP matches, the issues are invisible, but for roleplay, questing, testing and campaigning, it's an obvious and annoying bug.
https://youtu.be/-nUf7gpPLLM