I used to keep my phone in my back pocket but it has started to make a dent/hole on the seat where it protrude from my pocket
I dropped my phone while cycling a few years ago when I rode over a bridge dilatation joint and I've since wrapped an elastic band (a small slice of a bike tubing actually) around the holder and phone and it never dropped since then. I'll probably do the same here. Might as well add a lanyard attached to the ROP just to be on the safe side in case the magnet gives way.