Agreed on Belma not being the case if you're getting the classic strong movie butter popcorn kind of flavor. Maybe they caught a bug at bottling? Nov/Dec 2012 edition of Zymurgy has a good article about beer spoilage organisms and notes that some strains of pedio can cause big diacetyl flavors. How are you bottling out of keg? I have yet to work out a good method for this. I will typically just drop pressure on keg and fill straight from tap if I am filling a growler, and I have done bottles this way also with ok results, but I just got a good look at my tap the other day and its due for a serious breakdown and cleaning, so probably time to throw together a Biermuncher ugly junk bottle filler.