The IMAP mail protocol *guarantees* that incoming messages are filed by the server in the order received.
And AquaMail (and K9 Mail, and Gmail app for non-Gmail accounts, and many other mobile mail apps, which try to sync a limited number of recent messages) -- depends on that.
Now during this process of migrating user accounts to the new servers, this proper message order becomes messed up.
An oversight on Microsoft's part. Once the account is moved, your messages are not sorted by date (on the server) anymore.
Now as to why it's not the default in AquaMail, at least for Hotmail accounts -- I actually don't know how many uses (%%) are affected.
My account migrated just fine, preserving the order. I suspect not everyone runs into this...
And then "sync by date range" is someone less efficient and how many messages "last 10 days" or "last 30 days" varies greatly from one user to another.