I'm a bit confused as I've had the same issues lately. Was the 70 degrees for two weeks not a diacetyl rest?
I brewed an Oktoberfest, fermented for a month at 50, raised to 60 for 4 days and I think I tasted diacetyl. Yesterday, I kegged a bitter, using wlp005, that had fermented for 10 days at 68-70. I tasted the same buttery flavor in this ale. Maybe my perception of diacetyl is wrong, but I definitely get a buttery popcorn feel on the roof of my mouth and tongue. I don't taste butter, but I haven't discovered this in my other ales.