Villagers spawn without a profession

The bug

Since 18w50a, an unnamed villager called 'entity.minecraft.villager.none' will randomly be spawned using the /summon command or spawn eggs.

How to reproduce

• Place a command block with the following command

  /summon villager ~ ~0.5 ~

• Place a button on the side of the block
• Keep pressing the button until the command output mentions the internal name path

Analysis

This villager is separate from the existing and named "Nitwit", and is not the new named, GUI-less "Mason" either, as can be observed from the command output in the screenshot.
It can be summoned directly using:

summon villager ~ ~0.5 ~ {VillagerData: {profession: "minecraft:none", level: 1}}