Something to remember when applying history is look at the why as much as the how or what. So pre- 1st C. CE the Roman Empire had:
Running water
Mechanical lifts amd elevators
Water and steam driven engines (though not used to the extent or purpose of the later Industrial Age)
A somewhat centralized government
A somewhat centralized and organized economy
Public works
Massive masonry engineering projects
Infrastructure projects (roads, bridges, civil engineering projects)
A centralized military command (at least compared to most other civs)
A network of reliable communication around the Empire (considering)
Public baths and a general sense of personal cleanliness
And so much more...
Forward a few hundred years and we have people living in wood houses framed in poop, mud and straw, living in offal, covered in dirt, dung and fleas and wondering what race of giants built that weird bridge that goes nowhere (aquaduct).
While local Lords raise small armies of personal soldiers based on who they like and what titles they've earned versus actual ability.
Hey, but the swords are amazing.
My point is that even while we advance we regress. The problem with most fantasy histories is that we forget that non god-like beings sometimes cause problems. (Attila, Ghengis Khan, Hitler, etc.) So, I think it's fine that some of these worlds don't advance, but they need better reasons I would say. I don't think any Empire has stood the test of 1000 yrs. without some sort of forking, split, civil war/strife. So, measured regression inside general progression is the key.