I went through something similar in my pool this year as I thought I'd try out Fiber Clear before I was going to be cleaning the filters at the start of this season (I only clean them once a year since they are oversized 340 square feet for a 16,000 gallon pool). I added 2 9-ounce bags with no detectable pressure rise and no change in water clarity after a week or two. It didn't cloud up as your pool did. I E-mailed the manufacturer about it, but what they wrote back didn't make sense saying that it coats the outside area as in the circumference and not getting into the pleats. When I mixed up the Fiber Clear in the skimmer it's pretty clear that the particles are very small so would most definitely get into the pleats so would likely take a LOT of Fiber Clear to coat the filter.

Anyway, I ended up cleaning the cartridges and I don't plan on using Fiber Clear during the swim season. The water is in great shape and this was just an experiment. I may try again at the start of next year, this time buying more bags or maybe I'll just skip this entirely. It's not necessary, but still a bit disappointing that it's not more obvious how much is needed for a given size cartridge filter.

And yes, your filter is oversized for your pool, but that just means you don't need to clean it very often which for me is a great convenience.