on one of my test site I have this problem also have it already updated to joomla 3.8.9 dev but it remains strange. I can just put the image in a subfolder.
but I make a new article on the administrator side and I want to go there
uploading a photo and putting it in a special folder does not work.
But I can edit an article via the frontside there I can upload an image in a sub folder.
sorry for mv english 't s from google.translate