RFH: unexpected reflog behavior with --since=

I'm trying to leverage the reflog to speed up our $dayjob build procedure
(it's complicated), and found unexpected behavior when limiting reflog
output with --since.

    git init reflog-test
    cd reflog-test

    touch a && git add a && git commit -m'add a'
    sleep 1
    touch b && git add b && git commit -m'add b'

    # add_b will be the time that b was added (email ends with '>')
    add_b=$(tail -1 .git/logs/HEAD | perl -e "print( <> =~ m/> (\S+)/ )")

    # It's reported correctly here:
    git log -g --oneline --since=$add_b

    # But after a reset no history isn't shown.
    git reset --hard HEAD^
    git log -g --oneline --since=$add_b

Is this a bug?  Of course everything is reported when --since isn't used,
but not so when limited with --since.

1.7.7.1.msysgit.0

Thanks - Eric