@franksmcb, the version in Groovy (4.99.12+dfsg+really4.23.0-1) is working fine. I will update the observation in the bug report. Can it be backported to Focal without waiting for 5.2 release?
Not related but what is 'SRU' that Lukasz was referring to?
As you may have seen the bug report comments, it will take some time for the issue to be resolved. If it is urgently required by you, you can install 'flatpak' version of calibre (which is at version 5.2.0). Open a terminal and type following ('c' is capitalized)
flatpak install Calibre
If 'flatpak' support is not enabled on your system, ref to https://flatpak.org/setup/Ubuntu/ for instructions to enable flatpak support.