If you spam shift + click or right click (The one that puts the item into the furnace) in a furnace that is working, the material (ex) iron) disappear. You can still get the item though.
Linked issues
duplicates 1
Comments 2
Duplicate of MC-29318
MC-173134
Furnace Gui bug
Resolved
Duplicate
0
Can confirm this for Minecraft 1.16.1 playing on a multiplayer server running the official server download.