To reproduce:

1. Install fabric-carpet-1.16.5-1.4.28+v210224.
2. Launch Minecraft.
3. Create a new superflat world with cheats enabled, consisting of a single layer of glass at Y = 0.
4. Look straight down.
5. Run /perimeterinfo ~ ~ ~ armor_stand, optionally replacing armor_stand with any non-living entity.
6. Failed to spawn test entity is printed to chat.
7. (Actual behavior) The test entity briefly flashes into view.
   (Expected behavior) The test entity does not flash into view at all.

Example screenshot:

That is actually intended, as that is the way it is calculates. Also it is a purely visual artefact, so it isnt rly that big a deal tbh.

btw - moving them to under the world was the way of 'removing it from view'.

it mimics the vanilla way of spawning entites based on the id, like /summon does. Problem is that this way you don't know the entity type until it is created using a factory method.

Could rewrite using registries and getting them from there, but I don't thing it is that necessary. An entity to spawn needs to be a mob, otherwise its little pointless. I should change the error message, cause its not true. Didn't fail to spawn a test entity, just the entity to test was pointless.