I can only reproduce this bug unfortunately. It happened when I did a clean mininal install of Kubuntu on the 23rd. Not seeing Thunderbird in the menu, I tried "sudo apt install thunderbird", to be told that it was already installed. In fact, the following were all installed: "thunderbird thunderbird-locale-en thunderbird-locale-en-gb thunderbird-locale-en-us". Running "sudo apt install --reinstall thunderbird" fixed the missing menu entry.
I've since downloaded via torrent another Kubuntu 20.04 ISO and tried it in VirtualBox. For me, choosing the minimal install option (whether I've clicked try Ubuntu first and loaded the desktop, or clicked try Ubuntu now) still leads to no Thunderbird entry in the menu.
I can only reproduce this bug unfortunately. It happened when I did a clean mininal install of Kubuntu on the 23rd. Not seeing Thunderbird in the menu, I tried "sudo apt install thunderbird", to be told that it was already installed. In fact, the following were all installed: "thunderbird thunderbird- locale- en thunderbird- locale- en-gb thunderbird- locale- en-us". Running "sudo apt install --reinstall thunderbird" fixed the missing menu entry.
I've since downloaded via torrent another Kubuntu 20.04 ISO and tried it in VirtualBox. For me, choosing the minimal install option (whether I've clicked try Ubuntu first and loaded the desktop, or clicked try Ubuntu now) still leads to no Thunderbird entry in the menu.