Global match and capture group in look-ahead == under-populated "@-"?

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

use strict;
use warnings;
use Data::Dump 'dd';

$_ = 'a';
dd s/(?=(a))/b/;
dd @-;
dd $_;

$_ = 'a';
dd s/(?=(a))/b/g;
dd @-;
dd $_;
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Is this a bug?

1
(0, 0)
"ba"
1
0
"ba"
[download]