Hey! I’m from the same forum as @seanblue and have noticed this issue with details boxes being open.
I understand that the editor is apparently functioning as intended. However, it’s not obvious on the user-end that this is how the editor and details boxes are intended to function. If it was obvious, then everyone would be manually closing their details boxes and there would be no problems. 
We have a lot of users on the forum who aren’t used to using Discourse/forums at all, and they have a lot of trouble just figuring out basic functionalities like adding tables and details boxes to their posts to begin with; it adds a further point of confusion that the details boxes are not hiding information, especially with the flavor text being ‘This text will be hidden.’
Also, long-time users are not aware of the change, and suddenly details boxes are not behaving like they always used to, resulting in them randomly being open or closed because users have not realized there was a change. So this is confusing both to new Discourse users and long-time Discourse users. I’m really not sure who is benefiting here.
Then, there’s also the issue seanblue mentioned which is that we mainly use details boxes to hide spoilers in book clubs, and now that they are suddenly not closed by default, so when you open a thread, all the spoilers are visible, which is irritating 