Can you please explain, step-by-step, what you have done on what date?
If you reinstalled the app in January and connected to the sync and it downloaded the database with December missing, then this sounds like the sync was not working starting in December.
You can also check the modified timestamp of the database file on OneDrive (in case you haven't done any changes in the meantime, which I think you have done).
You cannot import the sync files from OneDrive, they are for the sync only. You can only import the manual backups, that you can create in the settings