On a device that uses roundIcon, that roundIcon will always be used (you’ll note that there are absolutely no APIs in PackageManager that allow you to choose between loading the roundIcon vs icon — that choice is always made for you).

All devices that use roundIcon have a manufacturer provided icon mask that is also circular, keeping with the contract of what roundIcon implies and that’s really what you should design for.

As you’ve found though, this means that if a launcher does include an option to change the icon mask (such as the Pixel Launcher’s Change Icon Shape option for devices with Developer Options enabled), it still uses the roundIcon as the base.

Personally, I’d take this as a strong opportunity to unify your icon and roundIcon into one glorious adaptive icon that designed specifically for adaptive icons. Google Translate is a great example of really embracing adaptive icons while keeping their traditional product icon for use in marketing, etc.