Okay certainly not the expert, but I just got through watching this morning apparently a podcast from the “fat bee man” that was recorded on YouTube, where halfway through it he said something that reminded me of a conversation/Posting from yesterday about adding boxes
If you add boxes to soon especially Honey supper, the bees need to keep the brood warm. And they build towards the top where heat rises. If you put in a honey super and a queen excluder the heat will escape to the top potentially as he called it creating chilled brood i.e. killing the larvae
This will set back the hive dramatically, this all makes more sense now to me.
Posting this one to see others comment and also to educate/Help others if this is truly the case