If you're cooking just one bird, take the free rebar out. The extra vent opening will give you better airflow for bird cooking temp.
The bottom vent does next to nothing for dialing in temp. Top rebar vents are your keys to dialing in temps.
You might ask why? Ok, same reason that the temp will increase drastically when the lid is crack opened, air drag.
Same applies to dialing in temps on the WSMs. Use the lid vent, but I digress.
Are your thermometers accurate? If your initial temp was 523 degrees, then the drastic drop in temp is airflow issue.
Try to pat down the foil as flat as possible or get a flat pizza pan to use as an ash tray