I have cats with this chronically. It's 'cat acne' - some are prone to it, kitties with allergies seems to be more prone. I wipe them with a human acne pad or witch hazel and let them heal - no antibiotic cream unless them get infected....vaseline instead.
The CAUSE of it is still unknown but here's more info on it at the links below. It's also never been proven that any type bowl causes it, only anecdotal evidence. Plastic does harbor more bacteria but if bowls are well washed often, bacteria doesn't have a chance to grow.
This article is a National Library of Medicine Pubmed entitled "
An evaluation of the clinical, cytological, infectious and histopathological features of feline acne."
Link here:
https://www.ncbi.nlm.nih.gov/pubmed/16515656
This is a Mercola (manufacturer) published article written by a vet. I'm not a big fan of anything published by a manufacturer. In this case, this is about the only article I could find that has a bit of 'meat' to it about the whole toxicology of the sores. Altho' this is still only the opinion of this vet in many places even the ending talking about plastic bowls. He can't quote any peer reviewed, actual research.
https://healthypets.mercola.com/sites/healthypets/archive/2013/12/09/feline-acne.aspx
I have 5 that have eaten twice a day from plastic bowls their whole lives - the oldest 2 are 10 years old. 1 of them had one once, never reoccurred. Snow eats off glass/ceramic/melmac (hard baked plastic) and has them often. Bebo gets one every few months. I wash all bowls/plates usually within an hour of them being used either by hand or dishwasher so they don't have a lot of chance to harbor bacteria. Those 2 that get them are my severe allergies/asthma cats.
I have spent a lot of time reading, researching and asking in all sorts of places about this topic, the cause is just not known
....and lots of time doctoring sores over the years.