This happened twice in the Chaos Wastes in the same run. I tempered a weapon and it disappeared from my hands for a short time. During this time I was unable to swap weapons or attack. (I was mashing Q and mouse1 button).
I’m not sure how to recreate this but I have a screen shot of the boons I had active at the time and videos of both incidents here: https://drive.google.com/drive/folders/1r0dUrl75ksGE06hN677fTn7SaQsRef17?usp=sharing
Also added the list of mods I had active but it’s a long list so it would prove difficult to identify if one of the mods is causing this.