This stopped working for my users as well just a few weeks ago. I created a separate forum which I will link below and got an unhelpful reply. There is a Microsoft Loop subreddit where people are experiencing a variety of issues, but this specific one appears to be incredibly niche, and Microsoft has not come up with a meaningful fix yet.
One person mentioned clicking the small message that appears in the top left and being able to click "Share and Notify" but that has not been my experience.