This Province is Too Close to Another Custom Nation

[Weebus]

When trying to add additional custom nations to a save file, you get an error if you try to add a province adjacent to an already existing custom nation. This seems unnecessarily restrictive to me and makes it very difficult to make huge fantasy worlds, as nations can't border each other, unless I want to do it all in one go without being able to save my progress. I just wanted to know if there was any reason for this and if it could possibly be looked at if it was necessary.

 

[Danfish77]

I feel like this restriction makes sense from a multiplayer balance perspective, but not in any other sense.
Why shouldn't two custom nations be OPM and neighbors in the HRE? I don't think there's anything engine-wise that makes this important, unless there's something very strange AI-wise going on with custom nations that I'm not aware of.