A file has two backup parts. One is data contents. The other is metadata, such as times and owners.
I think the warning is that the metadata that the file had at an earlier process point is no longer stored.
Exactly how this can happen isn’t clear. Are files with warning getting changed while backup is going?
Are they always VirtualBox files? Does your Options screen 5 set special options regarding backup?
The code looks like it handles an unexpected failed lookup of file metadata by storing metadata again.
You can watch the results of earlier analysis of your files with –log-file and –log-file-log-level=verbose.
Example:
2020-06-24 18:42:10 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FilePreFilterProcess.FileEntry-CheckFileForChanges]: Checking file for changes C:\PortableApps\Notepad++Portable\App\Notepad++\plugins\config\AutoSave.ini, new: False, timestamp changed: True, size changed: False, metadatachanged: True, 6/17/2020 4:17:08 PM vs 6/1/2020 11:14:06 PM
Usually you’ll see timestamp and metadata changing at the same time. Commonly size also changes:
2020-06-24 18:42:09 -04 - [Verbose-Duplicati.Library.Main.Operation.Backup.FilePreFilterProcess.FileEntry-CheckFileForChanges]: Checking file for changes C:\PortableApps\Notepad++Portable\App\Notepad++\config.xml, new: False, timestamp changed: True, size changed: True, metadatachanged: True, 6/17/2020 4:17:08 PM vs 6/1/2020 11:14:06 PM
If you can figure out a basic pattern to your warnings, above detailed view might help to further study.
EDIT:
If you search history of forum and GitHub issues, these used to happen a lot, but a change was done.
A quick attempt to search for 2.0.5.1 reports before this came up empty, which adds to its mystery…