Do you have MD+ set up to auto download investments by any chance?
Can you open console and split your screen to view the portfolio and console at the same time? What's occurring in Console when this happens?
I'm not certain how to control the auto-download settings on MD+, but MD does auto download with the"Quotes and Exchange Rates" extension. That said, I see the console that "MDP tasks" has a setting "forcedRefresh=true".
The actions I see in the console when the display changes are "uploading buffer file to syncFolder", "updating security" (from alphavantage) and then a mobile sync (via Dropbox). All appeared to be successful.
I am having a similar issues. During the past several days, sometimes the Portfolio View for an investment account shows none of the securities I own, sometimes it shows a subset of them, sometimes it includes ones that have been sold in their entirety. The view seems to refresh every 30 seconds or so and the securities listed may change. I believe the Net Worth calculation on the Summary screen may also be incorrect, perhaps related to this.
I do not use MD+ and do not download any investments; everything is entered manually.