–disable-synthetic-filelist is the manual’s word on what you’re missing. It’s a feature, but not essential:
If Duplicati detects that the previous backup did not complete, it will generate a filelist that is a merge of the last completed backup and the contents that were uploaded in the incomplete backup session.
The term filelist refers to what the UI would show as a backup version (file listing) for the Restore.
v2.0.5.104-2.0.5.104_canary_2020-03-25
Fixed storing
dlistfile after interrupted backup, thanks @seantempleton
is the fix, but it’s not in any Beta yet. What Duplicati version are you on, and was there an interrupted backup prior to this warning? On 2.0.5.1 Stop button, “Stop after current file” is the slower safer stop, however there have been some fixes to other ways of stopping (again, only in Canary release so far).
Even though you found the warning after backup, I would expect 19:26:40 to be closer to the start, as that’s the time when a synthetic filelist would be created (were it not for bugs). If subsequent backups are quiet, then I wouldn’t worry too much that you didn’t get the bonus of the synthetic filelist one time.
If by “exactly” you really want lots of details, please see Fix synthetic filelist not being uploaded #4114.