Is it a bug or intended that Unarmored defense from barbarian and monk overwrite each other?

For science I made a level 1 monk and level 1 barbarian character. Set all the stats to 18. So according to monks unarmored defense your ac = 10 + dex + wis. Then with the barbarian it adds con. Yet the ac was still only 18. Is this intended or is this a bug with dndbeyond?