Have you performed a test on the RO water itself? Don't "assume".
My phosphates shot sky high to 2 over time and I couldn't figure out the reason. After several different attempts to identify the source, I decided to test my RO water... It read 0.2 straight from the source. Almost a year's accumulation of ATO brought it that high. I ran GFO and brought it down again.
Alright, time to test my RODI bin today!
As a side note, what Phosphate test kit do you use? Currently using Salifert, but once I fix this phosphate issue I might go with the Hanna checker.