Welcome to the forum @romanodog
I think the intent is that a file gets scanned *if* it “changes”. An example of the testing process is below:
2019-06-22 11:39:53 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FilePreFilterProcess.FileEntry-CheckFileForChanges]: Checking file for changes C:\PortableApps\Notepad++Portable\App\Notepad++\backup\webpages.txt@2019-06-19_213642, new: False, timestamp changed: True, size changed: True, metadatachanged: True, 6/22/2019 3:20:41 PM vs 6/20/2019 1:42:25 AM
Viewing the Duplicati Server Logs at Verbose level might show you findings but could be swamped with output if you have many files. Advanced options –log-file and –log-file-log-level=verbose might be better.
Viewing the log files of a backup job will show you lots of statistics such as below. Can you post some?
ModifiedFiles: 3
ExaminedFiles: 422
OpenedFiles: 3
AddedFiles: 0
SizeOfModifiedFiles: 29902239
SizeOfAddedFiles: 0
SizeOfExaminedFiles: 402112849
SizeOfOpenedFiles: 29902239
You can see in above that in this backup only a few of the files seen were seen as modified, so scanned. Maybe your results are different, but statistics are a good initial way to see what other factors to look into.