When destroying a furnace block, the block entity persists for that exact position. This means that when you destroy a furnace that yields experience points, you can farm those experience points over and over again, as demonstrated in the attached video.

In this video, one spot in front of the player has a furnace with experience points in it. All other spots don't, and as such do not yield experience points. The player runs the command

/data get entity @s Inventory[0]

to show that the furnaces in the player's hand do not have any associated NBT tags.

This issue does not persist across sessions, so leaving the world or stopping the server will solve it.