I'm not quite sure what your question is. It's true that, with a custom order, if you decide to move some pictures around for a different order, you can't go back-- there's no memory of previous custom orders. So it's also true if you change to sorting by date saved or by date created, sorting up or down, that there is no memory of previous orders. If you had very quickly disconnected from iCloud, you might have saved the order at iCloud.com from synchronizing with your phone-- but usually it's too late by the time you've figured out what's happened.
Often, to avoid the mercurial nature of custom order, we add or change information in the pictures to help recreate an order. We can put a sequence number in the Caption for instance. On my Mac I add Titles (which don't appear in iOS) that have an index number. Most of us will change the dates and times on pictures so that they are put in an order that makes sense. I'm in the process of doing that now with some older pictures whose dates and times are flaky to begin with. But sometimes I change dates so the pictures will be in order to tell the story properly. The date and time are never actually changed on the original image file.
