Half my Pork Butt is done

Are you sure you didn't hit a fat pocket on the hot reading.A bone,even.

Are you cooking foiled?
If after rechecking temps in several places,you could tight wrap in foil and let the heat push thru the plateau.See how close to tender you get on both sides.Worst case, pull that you can and get the other up near 185º and slice.

Tom, I did not foil at all. I checked several places and the side with no bone read 199 each time. The probe slid through like butter. The other half (bone side) read at several places 174 and did not slide through like butter. I just pulled it from the smoker and did the FTC. I have never experienced a butt that varied 20 degrees from side to side. The butt was pretty proportionate as far as thickness.

I had a recent experience similar to yours cooking a brisket. One area was 25 deg warmer than another. It was weird but thats just the way it was. I think you are doing the right thing. Let us know how it turns out.

Spicey-meat,we aren't great fans of foiling shoulder/butts,but sometimes you try things-just to get the job done,as well as can be.

Sometimes,that tight foil,can help even out a bad butt and help salvage more.

What brand was it.

Different local meat houses may have different cutting standards and you get off cut product.

Walmart can sell some that are labeled strange and difficult to even identify the cut.

Keep cooking butts and it sounds like you'll be fine.

It is not uncommon for recipes to vary especially with meat or other organic products. Always compormise and adjust as needed.