I count on Quicken to keep additional, detailed data about many transaction. But it can't be counted on to preserve the detail captured in the "Notes" field. When Quicken erases this field, Not only is the data lost, but my time is lost, too. This is a reason why I dropped Quicken years ago, hoping that it would have been repaired by now as I began using it again.
I use Windows 10 and version R54.16 but that doesn't matter as this is an old problem spanning multiple versions on multiple Windows machines.
Another user has observed that this fault occurs when another field is entered after the Note is written.