This is a very large bummer. I need a bitmap mode with editing because I need to regularly touch up 20000 — 14000px size 1-bit 1200dpi line art files. Mostly it is just taking off a scrap that was in the original, and using an external software for that takes a lot away from Affinity's great concept of having everything instantly on hand in one screen.
I think opening a 1-bit .tiff received from the scanner, converting it into a grayscale document and editing it, for example, with a soft eraser, will give too many chances for weird things to happen on export (even if 1-bit export is possible). However, the above-mentioned colour enforcing layer limitation with 1-bit tiff export could work, if the .tiff's file size stays in the same class as in import (this is important, hundreds of pages stack up a lot of space if the size multiplies).
I think if this is documented well, and the output is reasonable, I think at least the comic-making workflow wouldn't suffer too much, even if this "cheats" a bit. Hopefully it will not break anything. The fact that Publisher retains 1-bit .tiff files is a small silver lining, I can at least lay out my pages with it.