It seems the orientation mode PortraitUpsideDown doesn't work in iOS and is ignored. The other orientations are okay.
App changes to PortraitUpsideDown
App is still in the normal Portrait mode
This is a big issue for us as well. Happening on all our XF apps. Xamarin.Forms v3.4.0.1009999
Retested with most recent XamForms version and seems to work now. With the exception of iPhone X and up, but it seems that is something Apple has decided not to support anymore: https://forums.developer.apple.com/message/268015
Thanks @jfversluis