I searched and did not see this addressed. I am aware that you can sort observations from oldest to newest on the desktop version, but I don’t see this option on the mobile app (I use Android).
I have tried turning Unknown observations into a coarse ID on my phone, and unwittingly made the mistake of adding a coarse ID when the person was doing a large upload, just to have them put the finer ID down minutes later. I have tried just scrolling down for a while, but the mobile app seems to reach a limit of how many observations it will load in a given session, so it doesn’t work so well.
If you use the ‘Explore’ tab in the app with the filters ‘my observations’, ‘date start to end’ and ‘descending’ it ought to work. (I translated the terms from the Dutch version, but I think you get the jist.)
The main ‘observations’ tab can’t be sorted as far as I know.