I think it would be more useful to regard this as a StreetComplete/editor flaw than an OSM flaw.
If you set the surface before the path is split, it’s applied to all of them and many desktop editors have some sort of Ctrl-Select/multiselect support that makes this no real hassle.
Mobian developed for PostmarketOS? I feel like you are mixing something up here, as those are both distros. Maybe you mean Phosh?