Furnaces that smelted items before 1.13 are complete scammers! If you remove items smelted in a furnace prior to 1.13 in newer versions, you won’t receive any XP. If you wish to reproduce this scam bug, smelt items in a furnace in a version prior to 1.13 (e.g., 1.2.5, 1.5, or 1.12) and then take out the smelted items in the latest version.