I had the same problem - a default icon was showing up in the task switcher. What fixed it for me is removing the round icon from my manifest.
As per the documentation for Adaptive icons,
You must only use the android:roundIcon attribute if you require a different icon asset for circular masks, if for example the branding of your logo relies on a circular shape.
(emphasis mine)
My app does not need a different asset for circular masks - removing the reference to the round icon from the manifest caused Android to show the correct launcher icon in the task switcher.
