[2022] Why do I get EXP for candles?

You're getting XP because technically you are 'harvesting' the block. Checking the blocks.xml, I see the candle is technically made of "Mpaper", which is defined in materials.xml to give 2 XP. The devs could change it to a material that doesn't give any XP.

This may be a very low priority bug/exploit, but it doesn't hurt anything to call it out and make the devs aware of it. See the pinned thread for where and how to file a bug report, if you're interested in seeing this changed.