It will only increase with time, until all of the precursor (alpha-acetolactate) has been converted to diacetyl. Many possible reasons for diacetyl in beer (lots of online resources for this). They're mostly yeast-related issues, not malt issues. Using pilsner malt, or any other malt for that matter, doesn't cause elevated diacetyl. (You might be thinking of DMS?)
Edit: One cause of diacetyl is a Pediococcus infection in the tap line. Your beer might be clean, but if there's pedio in the line, you can get diacetyl from the pour. I'd perhaps start your troubleshooting by putting a clean tap line on the keg and tasting what comes out.