In the screenshots I have included, you can see that the first one is comparing belts, which shows correctly. When I then immediately move my cursor to then compare the off-hand item, for which I do not have one equipped (you can see the slot is empty), the off-hand compares again to my belt like it did previously.
This happens for other slots as well if you empty them on your character and do the same thing, not just off-hands. Tested with boots and it also sticks in the comparison tool-tip.
And just to re-emphasize, this only happens when you immediately go to compare from one to the other. If I just hover over the off-hand without a previous compare, it shows nothing to compare it to.
