Not sure what distance you have there but I would say if you can put your hand on the pex and hold it there after the frig has been on for a while you are OK. The back of your had will let you know if there is a problem. Remember the pex can handle Hot Water much hotter than your hand.
Now if you want more protection install a piece of sheet metal mid way between the burner and the pipe. Don't fasten the metal to any part of the of the frig that is hot or to the pipe. Let it float between them. This should offer a good shield. You are dealing with radiant heat and the metal will brake the link between the frig and the pipe.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles