Here is my solution - https://stackoverflow.com/a/66603876/403801. Basically it was related to a Debugger tab window so when I changed my running behavior to open a new window instead of a window tab I have not had this issue anymore.

Bump. Over a year later and still have same issue.

I try to simulate the downgrade by purchasing the item from within my app. This appears to work however the receipt still shows that the expiration date is unchanged for the original purchase and so I am showing both subscriptions ACTIVE despite the fact you can only have one active at a time. Looking at the subscriptions from the settings/sandbox area, this appears correct. Really wish I had confidence in this process but I don't. And cancellation date always shows empty so can't count on that either.