@WhiteNoise Got it, my bad I had found the problem with everything deactivated but had reactivated some during my experiments just prior to that screencast.
Here is a new recording. All plugins and custom CSS are disabled, and the theme is set to None.
The behavior still occurs. I walk through it step by step in the recording, showing the CSS snippet and turning it on and off during the recording showing the difference in behavior.
Note the behavior varies depending on where the scroll position is. I discovered while making the screencasts that once a sub-bullet is the top-most line in edit mode, triggering preview mode causes it to jump immediately to the top of the document when the preview is rendered, then back when edit mode is triggered again. This may not be the only time it occurs but it was the most dramatic so I used it for the screencast. Note also that the movement varies in some cases even when the scroll is changed only by a tiny amount. (one click turn of my mouse scroll wheel)
Also apologies for being a bit rambling in my prior posts, this is just an extremely frustrating bug because it was occurring seemingly at random, and I still don’t know if there are other cases where it may occur. 
https://drive.google.com/file/d/1NcEJ1Ambhgh8X7Kkd90y-7w5EwHz_xz5/view?usp=sharing