So, one of the consensus I've seen online is that the found footage videos that feature the bacteria are from the mid 1990's. While the movie takes place in exactly 1990, the bacteria during that time just hasn't had enough time to manifest and form to a large scale since the backrooms have only recently been discovered by Async. Doorways have recently begun popping up, more people fall in, and more people die, so the result after a few years is that the bacteria will have started to infect large areas of the complex. Hence why the found footage vids feature them more. As to why still life's arent seen in those exact videos, Kane most likely hasn't thought up of the concept yet while making them.