Biofilm is possible, but I think it may be premature to be very concerned about yours yet. I would be curious to see how your porter is behaving after another 4 to 5 weeks, that would give you the information.
Alright, I'm resurrecting this thread for the second time, but it somehow came to my mind just now that I still wanted to give an update on this, FWIW... (for those interested, you might need to read the thread again, starting from post #14, to get back into the context of what I'm talking about).
So yes, I did have a diastaticus infection in that second porter split-batch, albeit it was a very latent one. After another 4-5 weeks (meaning 6-7 weeks from bottling) there was absolutely no sign of overcarbonation. The porter tasted great, and for several months it was also my preferred version out of the two split-batches. From what I could tell, the carbonation level stayed pretty constant for at least 4 months after bottling. I however noticed the same biofilm at the bottom of the bottles that was also present in my prior batch of WY 3726 saison. That was already a fairly clear sign of a possible infection to me, especially since it was completely missing from the bottles of the other split batch, which never came in contact with the diastaticus yeast.
After approx. 5 months, I started noticing a slight increase in carbonation in the potentially infected split batch. This increase continued fairly gradually during the following months, until by the time I drank the last bottle, almost one year from bottling, the beer was clearly overcarbonated while also displaying an excessive dryness for the style. Even then it wasn't yet overcarbed to the point of gushing, though.
FWIW, the second split batch never showed any signs of overcarbonation.
Moral of the story for me: I enjoyed both batches of porter (almost) to the last drop, but I also learned that our diastaticus friends can be a tricky buggy indeed. The infection can be latent and not easy to indentify. I would have dismissed that possibility, if I had not kept some bottles of the porter for 6+ months, and/or I had not observed the biofilm (which, by the way, was barely noticeable: I had to look into the bottles and against a good light source to see that).
What are your experiences? Is such a latent diastaticus infection, which develops slowly over several months, really a possibility? Or did I rather have some other buggers in there?
In any case, after first reading and posting in this thread I cleaned and sanitized that fermenter again and very thoroughly, and luckily I did not have any problems for the following beers I fermented there.
However, for the next time I am using diastaticus I might really think about getting an additional set of cold side plastic equipment.