Do you happen to remember the name of the file? My guess would be that nextcloud doesn’t sync it, but it won’t delete it. It might be available locally on the machine you created it.
It seems to me that nextcloud does have some version control (depending on your version I guess): Version control — Nextcloud 15 User Manual 15 documentation
Did your file ever get picked up by it at all?
Circling back to the folder idea, what would happen to those notes stored in the “recently changed” folder? Do they ever get deleted? And when? What happens if you edit notes on different places, do you get two different notes for each change? How do you merge them?