First, I think theives tools weigh a bit much. 1 unit per? Though I guess there could be strategy involved not carrying 100 of the things around. But I digress. My rogue was heavily burdened so I decided to pass off some of these tools to another character. When I split the stack and gave half of the tools away, my weight didn't change. I had to take the rest of my split stack, give it to another character too, to lose the weight of the pre-split stack. Then finally I had to take the half I wanted back. I tried it with other items too and noticed the same thing
Just checking, but were you moving them from a backpack? I noticed yesterday that when moving items from backpacks, the weight wouldn't update on the UI immediately. When saving and then reloading, it updated.