Another thing to consider is how the beer was stored. I have had bottled beer show signs of diacetyl after only a couple of days at warmish temp (80 degrees) while beer that was stored cold showed no signs. Oxidation during bottle can cause diacetyl if the precursor is present.Exactly what I was going to say. I have had it happen, and had a bottle set aside so I could check after reading the sheets, and yes, it had diacetyl. The keg was still fine.
Diacetyl? I have a high threshold. I get it in a beer at high levels, but I often let the other judge comment on that, unless I detect a slick mouthfeel, or taste "butter".