On new Iphone 11 Max IOS 13.3.1, in Maps app, created a new Collection and many map Favorites, which were added to the Collection. App gave msg saying Collection created. Many favorites in that collection were pinned on map. Closed the Maps app because was closing out all open app. Re-opened Maps app and new collection and all favorites are gone. Only items listed under favorites are a couple of items from weeks ago that don't think even saved as a favorite.
Thinking I messed up something, I made mistake of taking the time to re-add the collection and all the favorites as needed locations stored for upcoming trip. Closed app – everything gone again. HOURS wasted.
Have repeated "add collection" several times and whenever re-open app, all are gone.