Strange one...
Opened an anvil.

Moved a map to the first Input Slot from slot

The 44 wheat in the adjacent slot shifted left.
Took back the map from first input slot.

• The 44 wheat in slot shifted left but shows a count of 1
• The retrieved map shows a count of 44

When I closed the Anvil GUI and opened my inventory it was ok and had the 44 wheat were there so it is only the Anvil GUI there is no ill effect on the inventory afterwards so fairly minor bug I guess.