I have been seeing this issue on my end as well for quite some time however only now to the point when I decided to check the forums to see if it was reported. On my end it seems to be linked to smart cleaning as I have it set to "Automatically clean on closing with notification" for Edge, IE, FF, & Chrome. I have it disabled for Edge Chromium. My default browser is Edge Chromium.
I just saw it was missing from the taskbar, I called up the application and then closed it as it showed on the taskbar. I opened Chrome (one tab) to google.com and closed it which showed the cleaning animation however no pop-up to show the task was completed and when I moved my mouse over the icon it disappeared.