Comment 3 for bug 1929314

I've had the same problem with just about every MySQL 8.0 upgrade on Ubuntu 20.04, and now again with 8.0.27. I had to kill both MySQL processes in a 2nd terminal screen to make the upgrade process continue in the 1st terminal screen where MySQL's upgrade was started. Simply stopping the MySQL server daemon beforehand is also not enough.

klojum@hp840:~$ ps -A | grep mysql
  17286 pts/1 00:00:00 mysql-server-8.
  17667 pts/1 00:00:01 mysqld
klojum@hp840:~$
klojum@hp840:~$ sudo kill -9 17286
[sudo] password for klojum:
klojum@hp840:~$ ps -A | grep mysql
  17286 pts/1 00:00:00 mysql-server-8. <defunct>
  17667 pts/1 00:00:01 mysqld
klojum@hp840:~$ sudo kill -9 17667
klojum@hp840:~$

And then Ubuntu's upgrade process continues.