Thanks for reaching out to us through the Xfinity Forum. Rest assured that we are here to help, and I hope you’ll pardon my delayed response. When you get a chance, can you look and double check the settings on both of your Samsung Galaxy S8’s?
I want to make sure that you don’t have Intelligent Wi-Fi > Switch to Mobile Data enabled on the S8 that is dropping Wi-Fi calls.
That feature will switch back and forth between cellular and Wi-Fi signals depending on the strength of the signals itself. Let me know what result you get.