Layer models produce layers with horizontal or vertical lines (or both!)

[JoyfulGen]

@fujinaga this is the issue for Paco!

The lines are usually extracted from the background and added to one of the layers, or vice versa. They look like this (in these examples, the lines are created by pixels missing in layer 1):

@kyrieb-ekat made models that produce these lines systematically:
Produces grid - Model 1.zip
Produces grid - Background model.zip

This is the workflow required to reproduce the issue:
Fast.json

Here's an image to reproduce the issue with:
B&W alphabet.png.zip

@kyrieb-ekat's issue has more resources and info if needed: #1209

[homework36]

This should have been fixed by #1218