Sure! I think that would be a useful feature.
So an update... I tried it out first on the parent hosted feature layer. What I did from "Sort Options" was I changed the field to "Date" and sort order to "Descending", to show more recent records first. I changed one field to test it out and when I went back to the web app where the related records are displayed in the pop-up, no changes seemed to stick. Then I went to the web map and tried those changes, still didn't stick.
Then, on the web map, I decided to just apply those changes to all the related tables (~15 of them) and saved the map. I went to the web app, no changes stuck. I went back to the web map and looked at the "Sort Options" again, and most of them had reverted back, so it showed "ObjectID" for the field and "None" for sort order, except for the top one. It was really strange. Is that supposed to happen?
I also checked the related tables' attribute tables, and I sorted the same Date field in descending order, and it worked, so I know that the problem isn't based on the data type or entries or anything like that. It's just nothing is sticking! Do you have any insights on why this might be happening?