I was working on a mod that uses proxied magazines on a magazine pouch, which resulted in a bug when reloading a rifle. Video example below:
In the config.cpp, the magazines that are compatible have been modified to accept the new inventory slot. When testing in-game, you are able to drag a magazine from the custom slot, to the rifle and it reloads properly. You are also able to drag the loaded mag onto a stored one in inventory, and it swaps correctly. When you drag the loaded magazine into an empty custom slot, there is no reload animation, and the rifle bugs out. You can still tap R to cycle a single round out of the chamber, and you can shoot, but can no longer reload any magazine.