All Photos are suddenly sorted as oldest first—got a fix?

As léonie points out, "All Photos" provides a specific view of all the pictures in your Library in a pattern that can always be counted on--it can't be changed. But you don't have to "move them all to albums" to manipulate them. In fact, you don't exactly move pictures to albums-- any picture in an album is still in the All Photos view-- hence the name. An album is just a list of those pictures that you want to appear together, and pictures can be a many lists-- in many albums. But they're always in All Photos.

To do what you want with the least disturbance, you can make a Smart Album with a specification like {"Filename" "includes" "."} Since every filename has a dot, then you will get an album with all your pictures, and you can change the order. (I also use {"Date Created" "is after" "7/4/1776"} which seems to include them all!)

But in a Smart Album you can't order the pictures randomly-- the order must be alphabetical or chronological. However, in a regular Album you can drag the pictures to any order you want.

That is the way "All Photos" is supposed to be sorted on all recent system versions. Did you just install a system upgrade?

"All Photos" has a fixed sort order, that cannot be changed.

You can only change the sort order in albums you create yourself.

The following are the sort option for the various views in a Photos library:

iPhoto was the same.