SSTP has the option to actually set mppe by:
set vpn sstp ppp-settings mppe deny
This does not however seem to make any changes at all to the configuration in the file:
/run/accel-pppd/sstp.conf
I still have some problems left when I switch to radius authentication since it seems to get configuration information from radius (other than authentication information) - but this could be by design and I will need to look into this a bit deeper.