I have the same behavior here. I have a demo shop and a self made shop.
I am quite new to Magento so i am trying of lot of settings.
I do not have clear answers.
When I invalidated all the indexes and as a second step updates all the indexes by schedule the problem was solved. So my ques is could it be something with the indexing? Hope it sheds some light.
My question would be. Is it wise to invest in a indexing extension? for instance Improved Asynchronous Reindexing for Magento 2 by Mersavit?