I have analyzed the minimap "Summer Island", because it was the only track where my tools can't make the minimap visible in the center. Then I have found, that the track is very high. After lowering (and some other fixes) the minmap is now visible.
Then I have generated this OBJ file for tests: http://szs.wiimm.de/download/obj/50000.obj
It has long and short ramps from height 40000 to 60000. After creating a minimap with the CTools BRRES-Editor, half of the ramps (height<50000) is visible in the minimap.
My question: Is it a general issue of MKWii, or only one of the BRRES-Editor?