I am referring to this window:
It appears when Anki detected an error (especially an error in an add-on’s code) at startup.
The Windows 10 taskbar doesn’t detect this window as an Anki window; rather, it is not represented in the task bar at all. Here you can see that the Anki icon doesn’t have a blue bar when the window is open:
It should look like this (blue bar under the icon):