I have looked in the game folder. It seems like the game is using Haxe.
I don't really know what Haxe is but in the folder there is also Northgard.exe
and several .dll files. So I assume that Haxe does something comparable to
Wine.

Already having Wine 3.7 installed for another game I just ran: wine
Northgard.exe and bingo, the game seems to run just fine - at least the map is
OK and I could do basic stuff like building a house.

So assuming that Haxe is comparable to Wine and does run the Windows-executable
as well, we might conclude that the data that Haxe passes the mesa drivers is
not identical to the data that Wine passes them.

As a result I am quite sure that the key to the answer is in Haxe and not in
the game executable.