Ah, you are using a not yet public map, called "Randomized nonogram map" 😅
I was trying with "https://www.warzone.com/Map/41759-Nonogram" which probably explains the error. Was too late last night for me to realize the map did not at all match your screenshot :D
Now it also work in SP for me, and I can't immediately see why MP would not work. One thing I would try, is using -2 and 0 instead of WL.PlayerID.AvailableForDistribution and WL.PlayerID.Neutral. Not that this should matter thru, according to documentation both work. But I used -2 before, and I think I picked -2 for some reason. (Can't remember why thru.) Will try to check some more later today, just got to see if I can still set up AISever Bot for some free MP Mod test games :) https://www.warzone.com/Profile?p=7263394758
ps. I'll look into the master/main issue later on
I reported it to Fizzer, so I would not bother looking more at it. In practice it's just a UI error that new modders might run into (You already point to /master github, so that part looks good as well).