After fixing my local bug I rechecked the "*.sort{ a, b -> a.y == b.y ? -a.y <=> -b.y : a.x <=> b.x }" variant. Same result/conclusion.
However, on a hunch, I split in into two separate consecutive sorts. "*.sort{ a, b -> a.x <=> b.x }.sort{ a, b -> -a.y <=> -b.y }" So far this seems to do the job of fully sorting my 2d/coordinates set. (so far: no guaranties, as the used data set is somewhat limited and specific.)