For anyone still facing this issue, I solved it by changing the tracking option in the settings for my browser (Edge in my case) from Strict to Balanced.

Refreshed the Model Driven App in the browser. This resulted in the "login to flow" option disappearing from the "Flows" menu item.
I was then able to run the business flow (after logging in to Flow) where previously I had the Blank Log in to Flow popup.
If your company policy requires Strict, you could add exceptions to the profile for [*.]dynamics.com (part of your environment url) and [*.]powerapps.com:

Cheers,
Arjan