Run into the same issue on those old FR4 boards with big ground planes. The copper just sucks the heat right out of the tip, so a lower temp leaves you sitting there forever and the flux burns off before the joint even flows. What tip are you using for that 350C, a big chisel or something smaller? And does the board ever show any signs of delamination or lifted pads at that temp, especially around through holes? Trying to figure out if the heat is what fixed it or just the faster dwell time.

Boy, I was wrong about that show, totally hooked now.

Skip the spatchcock. It sounds smart but you lose the skin on the bottom and it dries out faster flat side down. Plus a flat bird takes up more rack space than a trussed one sitting upright in the basket. Trussing is annoying for 30 seconds but a 6 pound bird will fit if you tuck the wings back and tie the legs tight. The real trick is dropping the temp to 325 and adding 15 minutes, not changing the shape. Everyone acts like spatchcock fixes everything but it just makes a different set of problems.