Hi Darla,
If you are using Keep Everything it is true that the records will be maintained. If you have your Sync table set to Match Exactly from the source then if a Sync does run and there is issues with the source or no file is in the folder you are syncing too it can cause synced records to be deleted. That can also then cause the unconnected fields to be cleared out since the entire record gets deleted. If you are using Match Exactly and are concerned about data being lost in unconnected fields in the past I have put my unconnected fields in a child table instead so that they won't be lost just disconnected till the data is synced back in. I hope that information is helpful.