To be honest, I’d completely forgotten about that (Easter content in v.77). But it’s too late now. Next year.
The zones appear as the screen zooms out to encompass the whole wave (which is at 150% zoom). If the previous wave’s zoom was less than 150%, then they will initially appear just outside the visible screen. They will eventually come into view, however (barely, just before they disappear)
These are miniboss-class enemies, but Squawk blocks only include regular enemies. If I allow them, you could get into a situation where the entire wave is made up of Gatling/Lasers, which I’m sure would not be pleasant.
Changed in v.107 
Idea
Bug. It’s been fixed in v.107:
Added to v.107 Idea
Fixed in v.107 Bug
Fixed in v.107 Bug