As asked, I have removed the Enigmail add-on installed from the add-on store, clased Thunderbird, installed Enigmail 2:2.1.2-0ubuntu1 from the Ubuntu archive and the problem persists, now in Thunderbird 1:68.1.2+build1-0ubuntu1.
I still can’t reproduce the issue with the Thunderbird version downloaded directly from Mozilla.
As asked, I have removed the Enigmail add-on installed from the add-on store, clased Thunderbird, installed Enigmail 2:2.1.2-0ubuntu1 from the Ubuntu archive and the problem persists, now in Thunderbird 1:68.1. 2+build1- 0ubuntu1.
I still can’t reproduce the issue with the Thunderbird version downloaded directly from Mozilla.