Adding/Deleting items does not flush preview cache
It seems that Preview Mode does not update unless you modify a stack's settings.
In the example video, below, I have an Accordion I added to the page. I go in to preview mode and all is fine. Switch back to Edit Mode and delete the Accordion stack and then re-Preview the page again and the Accordion remains. This happens when adding content, as well, with the new content not showing up.
If you go into a stack though, in this example the Accordion stack, and make any adjustment to its settings, then the page will then appear as it should in preview mode.
Video example: https://d.pr/v/IQwaRz
At first I though this was happening as I was working on a .devstack
and they don't update properly any more as they should, but I don't believe that it is related to that bug.
Currently using Beta 17 here.