It's not only in the map editor,  you see the same thing in the game.

Any type of artifact.

All articfacts in my archive are reseached. One of them is different every time I reload.

Regulart artifacts on a custom map. My creatures revealed them.

Related tothis bug.

It's was working properly at some point. In current version of the game any gateway tooltip says it's destructable. I downloaded map "Blaise End" from workshop and it does not have this bug.

It's a rare bug. You won't get it more than a couple of time per mission.

You can reproduce the bug on this map. Buggy bombards don't just miss, they fire in opposite direction.

I think it happens when a beast comes out of arena.

Another bug: wardens ( attracted by forge) count as units attracted by barracks.