rakudo-j bug `my Blob $a = "a".encode; my Blob $b = "b".encode; $a ~= $b; say $a.perl;` + debug output

Buf.pm

# https://github.com/rakudo/rakudo/blob/nom/src/core/Buf.pm#L395

multi sub infix:<~>(Blob:D $a, Blob:D $b) {
    my $res := ($a.WHAT === $b.WHAT ?? $a !! Buf).new;

    say "\$res: {$res.perl}";
    say "\$a: {$a.perl}";
    say "\$b: {$b.perl}";

    my $adc := nqp::decont($a);
    say "\$adc: {$adc.perl}";

    my $bdc := nqp::decont($b);
    say "\$bdc: {$bdc.perl}";

    my int $alen = nqp::elems($adc);
    say "\$alen: {$alen}";

    my int $blen = nqp::elems($bdc);
    say "\$blen: {$blen}";

    say "nqp::splice({$res.perl}, {$a}, 0, {$alen})";
    nqp::splice($res, $a, 0, $alen);
    say "res after first splice: {$res.perl}";

    nqp::splice($res, $b, $alen, $blen);
    say "res after second splice: {$res.perl}";
    say '====================';

    $res
}

debug-output.log

# this shows the error is during the first nqp::splice

nickl@li685-90:~/.rakudobrew/jvm-nom$ perl6 -e 'my Blob $a = "a".encode; my Blob $b = "b".encode; $a ~= $b; say $a.perl;'         -------------------
$res: utf8.new()
$a: utf8.new(97)
$b: utf8.new(98)
$adc: utf8.new(97)
$bdc: utf8.new(98)
$alen: 1
$blen: 1
nqp::splice(utf8.new(), a, 0, 1)     # `{'a' is really utf8.new(97) but .perl-d}
java.lang.RuntimeException: This type does not support positional operations
  in block <unit> at -e:

gistfile1.txt

# I try to emulate the code in the infix sub from Buf.pm, but the error is not reproduced

nickl@li685-90:~/.rakudobrew/jvm-nom$ perl6 -e 'use nqp; my $x := utf8.new(2); my $y := utf8.new(1); my $z := ($x.WHAT === $y.WHAT?? $x !! Buf).new; my $x2 := nqp::decont($x); my $y2 := nqp::decont($y); my int $xlen = nqp::elems($x2); my int $ylen = nqp::elems($y2); say nqp::splice($z, $x, 0, $xlen); say nqp::splice($z, $y, $xlen, $ylen)'
utf8:0x<02>
utf8:0x<02 01>