If you swap your items with an item that can be placed in the offhand (shields, fireworks, totems, maps, arrows, and nautilus shells.) will disappear as you cannot put any other items in the offhand slot besides the 6 items which can be placed in the offhand. This affects survival and creative.
To Reproduce (This cannot be reproduced in Windows 10 unless if it's possible)
1. Put the offhand items in the offhand slot
2. Drag them or swap them with an item which players cannot place in the offhand
Offhand items disappear when trying to swap it with an item which cannot be placeable in offhand
Offhand items shouldn't swap with unplaceable items in offhand and should not disappear.