I added a debug log message to ptxd_install_fixup_timestamps() found in scripts/lib/ptxd_make_xpkg_pkg.sh to show the timestamp being used:

ptxd_install_fixup_timestamps() {
    local timestamp="@${SOURCE_DATE_EPOCH}"
    local touch_args
    if touch --help | grep -q -- --no-dereference &> /dev/null; then
        touch_args="--no-dereference"
    fi

    echo "ptxd_install_fixup_timestamps ${timestamp} ($(date --date=${timestamp}))"
    find "${1}" -print0 | xargs -0 touch ${touch_args} -c -d "${timestamp}"
}

I am currently using ptxdist-2019.11.0, and I get the expected result when directly printing SOURCE_DATE_EPOCH:
$ ptxdist print SOURCE_DATE_EPOCH
1572566400

$ date --date=@$(ptxdist print SOURCE_DATE_EPOCH)
Fri Nov  1 00:00:00 UTC 2019

However, the debug output I added to ptxd_install_fixup_timestamps() produces this output that corresponds to the current datetime:

ptxd_install_fixup_timestamps @1587399767 (Mon Apr 20 16:22:47 UTC 2020)

-Jon