a) screenshot shows v2.37b which is buggy in this relation
Well, I was going by the fact that since no fix is listed in History of Changes, v2.39b is equally bugged. But trying again shows your $num(%_counter%,3) does now work - thanks!
But still I get INcorrect output from $num(%_counter%,2). An extreme example is $loop(%_counter%):
I'd expect the output to be (string) sorted by track#: