If your facial recognition isn't working on a Surface Pro (especially after an upgrade), it might be because you are running the N or KN editions (Check in Control Panel!).
If this is the case, go install the Windows Media Feature Pack (make sure you install the one for the correct version you have), and this should fix it. If you do an in-place upgrade, for example to 1903, you will need to install the correct Media Feature Pack (again) to fix (again!).
Annoying that the upgrade doesn't also upgrade this optional component but meh.