Good plant growth, but Phosphates crash to 0ppm within hours of dosing

(High Tech) Recently setup a 50 gallon cube with overflow to a sump below.
Skipping to the point; I cannot seem to keep any level of phosphate in my tank.
Substrate is Fluval Stratum and there is quite a bit of it. (over 8" near the back)
I add Seachem Alkaline buffer daily to keep KH at 1dKH.
Once tds go above 140 I will do an RO water change with Equilibrium, and more Alkaline buffer.
Is this substrate known to absorb Phosphates?
I brought levels up to 2.0ppm after lights out, and they are back to 0.0ppm before lights on.

Sump has filter floss, bioballs, and 2L of Matrix.

Not sure if I should keep increasing phosphate doses, or stick to small daily additions.

I will go to small daily additions. How are your plants doing if they are thriving that's means is working no need to increase . I haven't kept (High Tech) plants for years now. but that is what I will do. Fluval is a great substratum second only to Amano's ADA.