Dear maintainer, I'd like to inform you that for some days, I can't observe these symptoms anymore on my Sid box; Enigmail is again able to fetch unknown GPG keys and display a green banner once the signature is verified.
Since there were no update of Thunderbird or Enigmail recently, but there was a GnuPG update on 2018-06-09, it seems it was indeed a GnuPG bug after all. As far as I'm concerned, this bug can be closed. Regards, -- Raphaël Halimi
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