Ok. I see what I might have made wrong: trying to resurrect the session (pressing 'r') 
I was able to reproduce it just this way:

After it asks for upgrade:
``` 
If you select 'y' the system will be restarted.                                                                                                                                                                                               
                                                                     
Continue [yN] y                                                              
I: molly-guard: reboot is always molly-guarded on this system.       
Please type in hostname of the machine to reboot:                            
Good thing I asked; I won't reboot upgteste ...               
=== Command detached from window (Wed Aug 19 21:24:36 2020) === 
=== Command terminated normally (Wed Aug 19 21:24:46 2020) ===
                                                             
Reading cache                                                  
                                                                       
Checking package manager                        
Reading package lists... Done                                          
Building dependency tree                                
Reading state information... Done                      
Hit http://ports.ubuntu.com/ubuntu-ports focal InRelease    
Hit http://ports.ubuntu.com/ubuntu-ports focal-updates InRelease 
Hit http://ports.ubuntu.com/ubuntu-ports focal-backports InRelease
Hit http://ports.ubuntu.com/ubuntu-ports focal-security InRelease
Fetched 0 B in 0s (0 B/s)                                              
Reading package lists... Done                                        
Building dependency tree                                             
Reading state information... Done                                 
                                                                   
Updating repository information                                                
                                                                                   
No valid sources.list entry found                                      
                                                                           
While scanning your repository information no entry about eoan could
be found.                                                          
                                                                          
An upgrade might not succeed.                                                         
                                                                              
Do you want to continue anyway?                                                           
                                                                       
Continue [yN] n

Restoring original system state

Aborting
Reading package lists... Done    
Building dependency tree          
Reading state information... Done
=== Command detached from window (Wed Aug 19 21:26:11 2020) ===
=== Command terminated with exit status 1 (Wed Aug 19 21:26:21 2020) ===
```
after that sources.list had bionic entries:
```
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://ports.ubuntu.com/ubuntu-ports bionic main restricted
# deb-src http://ports.ubuntu.com/ubuntu-ports bionic main restricted
...
```

I understand it may invalidate the bug.