Win 10 x64, Tiles, build 6784, https://imgur.com/orn06GM
If you try and add a UPS mod to a firearm repair kit, while having a stack of 3 on the ground next to you, it says “you attached your UPS mod to none” and the UPS mod disappears and the firearm kit does not change into a modded one. Seems you have to separate one from the stack in order to add the mod. I believe adding a modded firearm repair kit to the stack removes the mod as well, as I had one with a battery compartment mod, but it is no where to be found, which is how I encountered this bug after looking for my firearm repair kit to top off my guns, not being able to find it I decided to just add a UPS mod to one of my spares and this bug happened.
edit: found it, but still lost a UPS mod due to it being attached to none.