Missed OG bad w/ first use of Keg mash-tun...why

I've done a lot of AG brew using a converted cooler, I've alway have had excellent efficiency and usually come very close to my desired OG. I recently built a brutus stand and converted a keg to a mash tun. I did a repeat original recipe with an estimated OG of 1.065. I take very specific notes of my brews and did everything the same as I would with a cooler except that I stirred and recirculated the mash more and I fired my mash a few brief times to keep my temp regulated. This being my first attempt I overheated my mash raising it to about 163F about 25min into my 60min mash. My target temp was 156F. Using a refractometer, my OG was 1.048.

The last time I brewed this using a cooler I measured a 1.063.

Anyone using a converted keg have any advice or thoughts as to why I missed by such a large amount?

Are you fly sparging and do you have a false bottom or a manifold?

No I am batch sparging for 40 mins, same as I've done using a cooler. Yes I am using a false bottom