G'day nbtrap,

Welcome to the monastery.

I'm using 5.18.0. Without any context or knowledge of the values of $buf, $pat, $self or $sub, I am unable to reproduce your problem.

pos() and ${^MATCH} are returning the values I would expect:

$ perl -Mstrict -Mwarnings -E '
    my $buf = q{aXa};
    while ($buf =~ /a/gp) {
        say q{pos($buf):};
        say pos($buf);
        say q{${^MATCH}:};
        say ${^MATCH};
    }
'
pos($buf):
1
${^MATCH}:
a
pos($buf):
3
${^MATCH}:
a
[download]

perldelta (for 5.18.0) mentions a change to ${^MATCH} under Internal Changes which may be relevant.

perldoc.perl.org hasn't updated to 5.18.0 yet. If you're using online documentation, I suggest using the CPAN perl 5.18.0 doco. (The commandline perldoc utility is up-to-date.)

You may get some hints for creating a short, self-contained script that reproduces your problem from "How do I post a question effectively?".