The sync target really isn't suitable as a backup solution nor should it be used as such. A proper backup solution would be far better served by installing the simple backup plugin and having that export its backup to a cloud folder - or better, to both a local and a cloud folder.
I can only assume what has happened here is that for some reason the data in your sync target has gone wrong and caused Joplin to delete info and pull some notes in as a "conflict" when it isn't one in the true sense of the word - similar behaviour has been seen before when people have had issues migrating to new sync targets.
Is there any difference in data (number of files, most recent files etc.) between your local sync folder and what is actually stored in your google drive?
Although the data stored in the sync target doesn't consist of "true" markdown files (they contain all kinds of sync related metadata) you should still be able to search for content you had within your missing notes to see if they still exist on the target or not.
If you can't find anything then, providing you had note revisions turned on, you might still be able to find your deleted data in your database - although I'm not sure if this would have been deleted if the deletes happened as a result of Joplin deleting via sync.
Either way it is worth making a backup of your profile database in case you need to use the restoration procedure if all else fails.