DE coming through sand filter, is this normal/possible ?

[PoolDoc]

Am I missing something here ?

What you are missing is the fact that the 'design flow' is bogus. It's not your fault that you don't know that: the pool industry doesn't advertise the fact. But, there it is.

I don't know how they came up with that figure -- 25gpm/sft filter surface -- but it's unrealistic.

Years ago when they began replacing "slow rate" sand filters with 'depth filtration' "high rate" sand filters using "depth filtration", they were pushing the envelope with 15gpm/sft on a sand bed (in the Baker Hydro HRV) that was easily 18" thick. It worked -- not as well as the slow sand filters (5 gpm/sft) -- but it worked. Dirt retention in a sand filter is a function of sand particle size, flow rate per sft, and filter bed depth.

My experience, with many large (>100,000 gallon) commercial pools has been that filtration at 12gpm / sft is noticeably better than at 15 gpm. And it also improves if I 'overload' the filter with sand, increasing the thickness of the bed. When I installed my last set of commercial filters, in 2010, I used (3) 60" filters, loaded to a 30" sand bed depth, filled with gravel, then filter sand, then extra fine sand, then filter sand, and limited to 12-13 gpm / sft. With those filters, I routinely get 120' underwater visibility, in a heavily used commercial pool.

Your filter has about a 10" sand bed depth (1/3 of my big filters), running at 25+ gpm/sft, when you'd have to run at about 7 - 8 gpm to filter like my filters do.

That pump and filter package left the factory in as a sub-optimal design, 'broken from the factory'. To 'fix' the problem, you need to do two things:
1. Open the filter and replace any missing sand. Odds are, you've been losing sand on each backwash.
2. Install a valve or other means that allows you to throttle flow to about 15 GPM (PLENTY for a 1500 gallon pool)

Then retest with DE -- but use 1 cup at a time.