This has happened to me twice now, but the two situations are very different. Either way, the bug causes a map (in this case, a level 4 locator) to become a duplicate of my shield. I know it's a duplicate because the new shields have Unbreaking III and Mending, just like my original shield.

In the first scenario, it occurred while I was flying with an elytra. I switched from my shield to my map mid-flight, and the map became a duplicate of my shield.

In the second scenario, I exited the game, map in offhand. Maybe I had just switched to it from my shield, I don't remember. But later I come back to my world, and now I have a shield in my hand. I look in my inventory, and there's another shield, but my map has vanished.

Fortunately for me at both times, I had copies of these maps. But the second time I did not have a copy at my base, so I had to go to one of the villages on the map that I had placed a copy in. I didn't know the area as well since I'd just explored it, so it was very much an inconvenience. However, if I didn't have any copies of that map, this would've deleted all the work I'd put into that map, which would have made me particularly perturbed.