You have something strange going on, if it were me I would go ahead and raise the FC to SLAM level for 48 hours or so (72 would be better) and see if there is any improvement in how long your filters last before needing to be cleaned. I would not expect it to be solved within 48 hours, but that should be long enough to see if there is a trend of the filter lasting longer between required cleaning. Also check hidden nooks, light niches, under steps, etc. for algae growing (and scrub), sometimes algae hides in such places as they have poor water circulation which can make it hard to kill, and also somewhat hard to detect.