This does not surprise me ~ I have never had a twin cam rear head make it to 41k ~ This experience has been across an 06 SE ultra traded at 93k Rear exhaust Valve guide failed at 40 & 80k ~ a 10 LTD sold at 71k rear exhaust valve guide failed at 30k less than a year old. After the replacment on the 10 I had FM work a set of heads for me & no issues since. I have already sent a set of 110 heads to be reworked since experience has taught me that Next Summer I will need a set ~ All three failures I had were actually cruising done the interstate one on decel ~ turning of cruise to exit ~ and two on accel to pass.
IMO the heads are the overall weak link on the late model twin cams
2012 FLTRXSE 113 Build ~ Chopped Tour Pack, Lowers, 13" Tinted MadStad, S&S Power Tune Duals Jet Hot Black, Daymaker LED's, Adaptiv TPX Radar Detector, Fork Baffle, Pro940's & Monotubes, Baker DD7, Zumo 660