[pull-lp-source] not overriding default configurations

My Ubuntu version: 20.04.3 LTS
Package version: 0.188

According to the manpages, "pull-lp-source" script should parse environment variables or devscripts file to override package-wide variables.

However, this is not working for me. For instance, when I set the environment variables to:

    export PULL_LP_SOURCE_MIRROR="https://de.mirrors.clouvider.net/ubuntu"
    export UBUNTUTOOLS_UBUNTU_MIRROR="https://de.mirrors.clouvider.net/ubuntu"
    export PULL_PKG_UBUNTU_MIRROR="https://de.mirrors.clouvider.net/ubuntu"

    export UBUNTUTOOLS_MIRROR_FALLBACK=no

the script does not try to fetch the package from the mirror I have specified, and falls back to the default mirror even though I have specified it not to:

    $ ubuntu-dev-tools-0.188/pull-lp-source -v --download-only dash
    pullpkg options: {'login': False, 'verbose': 1, 'download_only': True, 'mirror': None, 'no_conf': False, 'no_verify_signature': False, 'status': [], 'arch': 'amd64', 'pull': 'source', 'distro': 'ubuntu', 'security': False, 'upload_queue': False, 'package': 'dash', 'release': None, 'version': None, 'ppa': None}
    Found dash 0.5.11+git20210903+057cd650a4ed-3 in jammy
    Downloading dash_0.5.11+git20210903+057cd650a4ed-3.dsc from ports.ubuntu.com (0.002 MiB)
    [=====================================================>]100%
    Public key not found, could not verify signature
    Downloading dash_0.5.11+git20210903+057cd650a4ed.orig.tar.xz from ports.ubuntu.com (0.127 MiB)
    [=====================================================>]100%
    Downloading dash_0.5.11+git20210903+057cd650a4ed-3.debian.tar.xz from ports.ubuntu.com (0.041 MiB)
    [=====================================================>]100%
    --download-only specified, not extracting