So draining it does the trick? Don’t need to run anything through the line? Makes me wish they super shock these things before they ship.
That water was certainly chlorinated when the spa built and whatever water was left would have had chlorine residual when it was wrapped up and no matter the level of chlorine, the problem is the weeks and months between then and you filling/firing it up.
You certainly found your problem with the phosphates. I'm not exactly chemist nor can I speak about chlorine generators (for spas or pools) and while I know phosphates have to be dealt with when high I always do a search to refresh myself and doing so you can find some simple explanations of the reason for the issue:
Chlorine Generator (Salt Pools)
Phosphates and calcium bond to make calcium phosphate, which causes ineffectiveness of the generator. By removing the phosphates and maintaining proper calcium hardness levels, you can keep the cell running at optimum performance.
Do phosphates affect chlorine?
Phosphates are food for algae. ... High phosphate loads, when left untreated, will generally result in higher chlorine demand and can even lead to algae blooms in your pool. A higher chlorine demand can tax, or even overwhelm, the production capacity of a chlorine generator.