I had FOV last December and did not have to lie face down. However, on 2 previous retinal tear surgeries, my surgeon asked that I sleep on my left side and sit with a leftward lean so that the gas bubble would lay against the repaired part of my retina to promote healing in the first week after surgery.
So,I think the post surgery “posture” requirements depend on your surgeon’s preference AND other eye conditions you may have that pose risk.
I’ve posted in this group lots of times. Good luck!