[MiWa]

How about the original maps, like Crimea for example? Are such features present if you use the tool on original map_c (create full map_c then chop it)?

I mentioned this earlier, but it wasn't noticed. There is another set of maps, which resemble map_c. They are almost identical, but seem to be using more shades of grey. I believe those maps are used by the engine because even the new maps from the latest patch come with those 'extra map_c'.
Maybe, those are responsible for this effect you are observing?

I'll post more details on these mysterious maps later when I get the data I have on another computer.