Honestly, not quite sure why it happens, but I know it’s something to do with using it in Korea, where Pixel phones are not natively supported. In other words, it happens with Pixel phones, regardless of the OS.
As to how I knew it would work with the fix, it was a gamble. I rooted my phone and enabled VoLTE prior to that app.
A couple of things come into my mind. Note that this isn’t exhaustive.