Thank you for answers. A bit carried away with "non-determinism", it's just random data in unrelated memory. And now I see why I couldn't repeat this bug with checker-board pattern -- rather something like right-leaning triangles are required:

my $bin = ( xvals( short, 1024, 400 ) % 4
          + yvals( short, 1024, 400 ) % 4 * 2 ) > 6;
say $bin-> info;
say $bin-> slice( ([ 0, 7 ]) x 2 );

say $bin-> cc8compt-> max;
say $bin-> long-> cc8compt-> max;
[download]

PDL: Short D [1024,400]

[
 [0 0 0 0 0 0 0 0]
 [0 0 0 0 0 0 0 0]
 [0 0 0 1 0 0 0 1]
 [0 1 1 1 0 1 1 1]
 [0 0 0 0 0 0 0 0]
 [0 0 0 0 0 0 0 0]
 [0 0 0 1 0 0 0 1]
 [0 1 1 1 0 1 1 1]
]

31344
25600
[download]

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