Thanks for the reply, Diego.
It is set to Auto.
The problem is also present in runtime, if that wasn't clear.
Also, TiledBackgrounds (and Sprites) at the edge of the viewport are disappearing. Which I think is relevant because I think that Construct calculates that they are out of sight, but as they are rendered further back, they, in reality, aren't.
I think this was a result of the deprecation of the old Z rendering mode I can't remember the name of in favor of "Normalized".
Edit: I have tested tweaking that option. I think "Auto" is setting it to "Yes" because "Yes" changes nothing. "No" completely screws up everything very badly.