The closest I could find is some very old information saying it would be available on Game Pass “in December.” I sort of make the assumption it will be simultaneous with the Epic release, given the Epic release is also stated as “in December.”
See this page - https://phoenixpoint.info/faq - under the caption " Which platforms will Phoenix Point be distributed on?"
See, also, this - https://phoenixpoint.info/blog/2019/11/22/november-development-update - " Microsoft Game Pass. Phoenix Point will also be available to play on Microsoft Game Pass on Windows 10 for those with the appropriate subscription. We are aiming for this release to happen simultaneously with the Epic Games Store release, but the exact timing depends on certification with Microsoft and is therefore out of our control."
That’s not incredibly inspiring, I know.