Wyeast 1318 repitch & attenuation

Pitched about 400ml of 3rd generation Wyeast 1318 (london ale III) into an old ale. OG was 1.099, mashed at 154, 8% crystal, even added a few pounds of DME to help keep attenuation low. Was hoping for a nice, thick brew in the 1.024-28 range. Instead, kegged it yesterday and it was down to 1.013…87% attenuation. Considering the yeast, mash, and grain bill I just cant figure out how it got so low? Taste is okay (ie. doesnt seem infected), heavy alcohol obviously at 11.3% abv. Other interesting thing is that it has a very belgian like, phenolic aroma…like you might expect from a strong tripel. Could still end up good, just definitely not what I was going for. Can a couple repitches change a yeast to this degree…its acting like Wy 3787 now.

Based off the phenolic flavors and unexpected attenuation there is a good chance contamination in the culture is to blame. Harvested yeast can mutate over time but not like that and not that fast.

Considered that, but it fermented down in just over two weeks. I figured an infection from contaminated yeast would work a little slower than that? Also doesn’t smell/taste bad. Its not medicinal or funky or nasty, just very belgianesque is the best way i can describe it. Maybe it’ll show itself as infected with a bit more time, its only three weeks old. Still kind of stumped by this one.