This is based on my experimentation, but this seems like it's related to exploiting the bug that lets you run crazy fast by punching while you run.

I've only ever experienced this problem after trying that. If I run/jog/sprint normally I do not experience this problem. I've intentionally tried this on poor ping servers (for me).

I've been testing this. Since the 19th I have never experienced this bug as long as I un-readied my current weapon or fists before trying to use the hotkeys to swap to another item.

Every time I try to swap with a "readied" or shouldered weapon this bug happens.

I think, It can be avoided by ensuring your current weapon is lowered before swapping.

I think the important difference is whether your currently equipped weapon is "readied" or not. Swapping with the number keys from a "ready" position (rifle to your shoulder) seems to cause the character to swap to an item and then to nothing. Swapping from the "down" position (rifle lowered) works appropriately.