Thanks for the response. Yes, I set sail for a new island at least once, and it did not save that unfortunately. I accidentally browed away from itch in my browser in that case, then discovered the game had effectively reset. Then I tried again several times with progressively shorter game sessions to try to diagnose it on my end (was it my browser settings? or maybe the Windows app worked?).
I had no idea that the game did not save until you left an island, though, so much of my testing was bound to fail -- after losing a lot of progress a couple of times, I was just testing very short plays to try to figure it out, without success.
It would be great to be able to save any amount of progress (not just post-sailing), or have an import/export save file at a minimum (even if it's just the local save data) so the the user can save as often as they want and recover progress in case saving errors in the game.