When I have a Chromium window open for a while, GNOME and/or just the Ubuntu dock loses track of it. Note in the screenshot how I have a window open but there is no orange dot next to Chromium on the sidebar. I can still interact with the window with the mouse, I can still Alt+Tab to it, but I cannot navigate to it with a click on the Chromium icon on the sidebar or using Super+1 (as it is on the top of the list), both of these actions open a new window as if it isn't currently running.
As on September 24, 2020, I was using Ubuntu 19.10 and GNOME Shell 3.34.1. But then later on, I upgraded to Ubuntu 20.04 and I'm still experiencing this issue.