Interesting discussion. I have noticed that I frequently have a Face ID failure when I first pick up my M4 iPad Pro 13 inch but that if I try a second time, FaceID works fine. This discussion got me to thinking why.
My conclusion is that when I first pick up my iPP, I hold it by the right edge because I don't want my Pencil Pro to fall off. Once I have a failed login using FaceID, I pay a tiny bit more attention to where my hand is placed (often without consciously noticing I am doing so) and Bob's your uncle... FaceID works the second time.
Hopefully, now that I'm aware it's not just me, being more intentional about hand placement will reduce the number of FaceID failures for me.
Implicit in the above is the fact that when I am using my iPP as a handheld tablet, I am typically using it to read or to handwrite, both of which work better for me in portrait mode.