Burnt valves, '08 Mercury Mariner 3.0 V6, similar to Ford Escape

Question

Seeing if anyone else has ran into this problem on a Ford 3.0 V6? It's an '08 Mercury Mariner, 150K miles. Well maintained. High quality synthetic oil used in engine. Almost if not the same as a Ford Escape of the same year. It was starting to randomly pop/backfire through the intake once warmed up at idle. Ran fine at anything above idle and performed just fine. Thought it may be a bad tank of fuel. Wasn't the case. Ran several fresh tanks through afterwards and no change. Pulled the plugs for inspection, they were fine. So, based on the cranking rhythm not sounding good, did a compression check. Had multiple cylinders low on compression, and one really low. Performed a leak down test and revealed the exhaust valve was leaking badly on cylinder #4 with the lowest compression. Pulled the front/left cylinder head off to find one of the #4 exhaust valves burnt badly, and the #5 and 6 cylinders not far behind. Obviously needing to rebuild/replace the cylinder head assembly (and possibly pull the rear head for inspection as well), but I'm wanting to find out why they failed. Doesn't appear to be an overheating issue. The head is flat as the straight edge I checked it with and not warped. Fuel trims were right at +5 and +10 respectively and no MIL , so no lean mixture suspected. Wondering if anyone else has run across this issue? Best I can guess is they ran E85 through it unknowingly and maybe that was enough to damage the valves therefor the fresh fuel after I started diagnosis wasn't going to have any effect. If you've any wise input I greatly appreciate it. Thanks guys!