Here's how to reset the layer pop-up to the default attribute order of its schema in the Map Viewer (i.e. reverse the automatic alphabetical sort on fields in the pop-up that seems to be the default when a new layer is added to a web map):
1) In the Map Viewer (not Map Viewer Classic), select the layer of choice from the Layers Pane table of contents (left sidebar)
2) Select the Pop-ups button from the layer settings (right sidebar)
3) Select the existing Fields list button with the fields currently in alphabetical order
4) Click the Select fields button (which will change to Deselect all)
5) Click the Deselect all button (which will change back to Select all) to turn off all fields
6) Click Select all once more—notice the order reset in the "dimmed" Fields list menu that appears behind the active Select fields menu
7) Click Done at the bottom of the Select fields menu
😎 To save these changes, don't forget to click Save from the Save and open menu (left sidebar)
Related idea: Show Pop-ups in Attribute order rather than Alphabetical by Default
