Not sure how best to describe their all purpose weapon, so I went with "gun". Anyway what's weird about this glitch is it doesn't seem to be caused by mining specific things. Sometimes I'd see it during water gathering (like in the image attached), sometimes when grabbing seeds, sometimes when grabbing food, a couple times when delivering copper ore... etc. So I honestly don't know how to reproduce this glitch.
It's also worth noting it only has been happening to this, specific, duplicant. None of the others. And, finally, I only noticed it after I had him use the medical cot but again, I've had other duplicants use that cot and no problems.
Apologies for not being more useful.
Honestly not sure. But yes, this is on the thermal update.