This is a tricky one.
I think it’s reasonable to assume that most people create details sections with the intention of having them closed upon posting to avoid cluttering or overpopulating their post with perhaps ancillary content; otherwise, why have the content in a details section at all?
But, if we default to closing all details sections upon post, then we make it impossible for anyone to post an open details section without switching to Markdown-mode and it does conflict with the premise of WYSIWYG. If it’s open in the editor, then it’s open in the posted topic / reply.
I wonder if the placeholder content is confusing — when it is open, we tell you “this text will be hidden”:
I don’t have a clear idea yet on where to go with this, but I agree something feels off.