This is a problem I'm running into now with the Maiden, but have also run into with every other model I've had to do this with...

Glue a sandwich fold. Let's say the scored part of that fold is the "top", since that's usually the top of the wall we're working with. Burnish it, let it dry.

Vertically score one side of the wall, such that the wall can be bent to form the corner of a room, and glue the wall to whatever floor you're working with via the tabs on the bottom.

Notice that both outer corners of the top of the wall bend down and "in", such that they're bending toward the corner formed by the fold along the vertical score and the floor.

This has happened every time I've tried making a model where you score a sandwich-folded wall and bend it (it's not a problem if there's no texture inside and it's just a single sheet of cardstock)... Does this not happen to other people? Is there something I'm doing wrong? If need be I can post some pictures to clarify...

Do you press the walls while drying under a weight of some kind?

My guess is either what I put above or a non-straight score line. The only other time I have seen that issue is when I printed some stuff on a laser printer at work that curled the pages really badly due to not being meant for cardstock.

Hmmm... Maybe it's that I don't counter-bend my pages as much as I need to to get the curl out of them. I'm using a Brother HL-4040CN, and the curl on the pages is pretty intense after they come out of the printer... I'll try getting them to lay a bit more flat and see if that helps...

I figured out what I was doing wrong... If you don't fold the wall past what it needs to be, 90 degrees for a 90-degree corner for example, it will bend the wall down. So if you've only folded it maybe 15 degrees to edge it, then glue it down at 90, the top won't behave the same as the bottom near the glued area. You have to crease the paper past 90 degrees for it to stay flat.