the first time i approached to sstp i had the problems, in fact never been able to make it working with self signed certificate
because of that we tryed buying a certificate from a recognized provider
since that day
we have used native SSTP client many years without any problem
but i make it clear
using a certificate from a recognized provider and renewing it every year (paying the respective fees)
maybe is not the solution you are searching for, sharing my experience is the only way i can contribute to this topic