This is a bug that has been around for years but is a bit weird. I ran the command
/execute as @e[type=minecraft:furnace_minecart] at @s unless entity @s[nbt=\{Fuel:0s}] run data merge block ~ ~-2 ~ {BurnTime:100,CookTime:20,CookTimeTotal:20} and it works but the furnace gui just expands part of itself it's kinda odd.
I provided an image of the bug.

How to reproduce
1. Run the command in a repeating always active command block.
2. Let a furnace minecart run 2 blocks above it on a rail and let it go buy.
3. Try to cook food with coal it might also work without.
Note this bug is not only in 1.16 but in many versions it may go as far back as the setblock command but I haven't researched it however I think I've seen it before.