Agreed. I was an early outlook.com user (was working at MS when it launched, I think internal users got slightly early access allowing me to claim a nicer name than my Gmail) but despite having well over a decade of accounts tied to it got so angry at certain messages never appearing that a couple of years ago I reversed the flow of forwarding and swapped to another account as my primary.
Sounds like it's gotten even worse.
I always thought of outlook.com as a rebranding of Hotmail (which itself had been continually evolving, was probably actually “Live” at that point), I would expect it is the same (ever evolving) infrastructure.
In which case, people like me with an @hotmail.com address from the 90’s were much earlier users of the outlook.com email boxes than when the domain was “launched” by Microsoft.