If you have a sword equipped and equip a shield in the shield slot, the sword will unequip and move to inventory.
Ok, I figured out how to get it again and can get a video if you need one.
It works fine in the My Deck screens of inventory. If you switch to My Heroes, it works fine with Squires that are in your deck. But if you change to a squire who is not in your deck, then the bug happens.
I knew I wasn't going crazy! :)
I haven't seen this interaction occur. If I have a sword equipped, both right clicking a shield and clicking and dropping a shield into the shield slot appear to keep the sword equipped. Can you make a quick video showing it happen so we can try to reproduce it exactly as you are seeing it?