SQL Server - Files timestamps unchanged so what is it that's being backed up?
In answer to your main question ("what is being backed up"), everything that is supposed to be   More seriously, you can't use those time stamps as a sign of changes happening in the database files.  SQL Server doesn't make any guarantees about how the modified date of database files will be changed or updated.

There is an interesting blog post here that attempts to find patterns with when and how the date modified stamp is updated on MDF files:

SQL Server Database File - Date Modified

Their observations line up with your situation I think: normal writes to the data file don't update the date modified stamp.  It looks like growth events (manual or automatic) would update that value, as would restarting the service (or anything else that might "open" or "close" a database - using the autoclose feature, explicitly taking the DB offline, etc).

I expect 19/12/2019 9:52 PM corresponds with a Windows or SQL Server restart, which is how several of the date / times got sync'd up.

However, again, this behavior is undocumented.  Any patterns we notice could change at any time, or have unexpected edge cases.

For SQL Server data files residing on an NTFS file system, this applies:


  The only guarantee about a file time stamp is that the file time is correctly reflected when the handle that makes the change is closed.


Though, to be clear, that's a guarantee provided by Windows.  We can't necessarily know a ton about how SQL Server creates and destroys file handles in every scenario.  

David Browne (a Top Microsoft Man) mentioned this in a comment, which is useful to know:


  SQL Server keeps the database files open whenever it’s running, for better performance and protection against other processes changing them. So the attributes don’t get changed on every file write.