Roast Whole Chicken frustration

I have had amazing results from Samin Nosrat’s buttermilk-brined chicken but I want to branch out a little, technique-wise and try to incorporate a wider range of flavors Lately I’ve been practicing spatchcocking chicken and I keep getting the most frustrating results. I’m cutting and cracking correctly and most recently I followed Sohla’s NYT spatchcock recipe (there’s printed and video to reassure me I’m not doing anything wrong) I’m not getting some mutant 6 lb chickens, these are all 3.5-4.5 pounders. Dry-brining, rubbing in ghee, bed of aromatics and roasting at 450 with the legs in the back. And I’m using a wireless probe (Combustion. Expensive. Works with the buttermilk chicken.) Every time, the legs get up to 190, parts of the breast are at 140 and other parts of the breast are at 110/120. This is not a one-time thing, this is an everytime thing when I spatchcock chicken. No matter how long I leave it in - sometimes 90+ minutes! What gives? submitted by /u/GunchiesGoo