With %60.6 apparent attenuation. Wow thats low. What was your mash temp? Perhaps the reading on the hydrometer at OG was off? Interesting. I would have piched in some 001 or some nottingham but thats just me. How do your other beers finish?
If it tastes good then I wouldnt worry and enjoy how it ages. Id be concerned if you do bottle to wach out for secondary fermentations. I do recall a Imperial Stout that Id bought that had a starting OG of 1.120 and finished @ 1.04. Corked bottle and was ment to be aged for 20 + years . No priming sugar. It only lasted a few months in my fridge before we consumed it. Sweet but quite a nice dessert.

