Não é possível atualizar pacotes

Question

No Debian 9 Thunderbird instalado experimental. Defina as prioridades definidas abaixo:

Package: thunderbird* lightning
Pin: release n=sid
Pin-Priority: 1100

Package: *
Pin: release n=stretch-backports
Pin-Priority: 400

Package: *
Pin: release a=testing
Pin-Priority: 300

Package: *
Pin: release n=sid
Pin-Priority: 200

Package: *
Pin: release a=experimental
Pin-Priority: 50

apt-cache policy thunderbird:

Installed: 1:52.2.0-1
Candidate: 1:52.2.1-3
Version table:
 1:52.2.1-3 1100
    200 http://ftp.pl.debian.org/debian sid/main amd64 Packages
*** 1:52.2.0-1 100
    100 /var/lib/dpkg/status
 1:45.8.0-3 500
    500 http://ftp.pl.debian.org/debian stretch/main amd64 Packages
    300 http://ftp.pl.debian.org/debian testing/main amd64 Packages

e o mesmo para relâmpago e thunderbird-l10n-pl

Existe uma versão mais nova no sid agora, mas quando tento atualizar ( apt-get dist-upgrade ):

The following packages have been kept back:
lightning thunderbird-l10n-pl

política do apt-cache:

thunderbird -> 1:52.2.1-3 with priority 1100

Como atualizar o pacote instalado de outro repositório?