"Patrick, photographic grade Potassium Sulphite is sold as a 45% w/w solution, so 384 will dissolve easily, but maybe not quickly."
How about overnight, starting in 160 F water? I stirred it for a good half hour before going to bed. That doesn't explain what happened here. This morning, I decanted clear liquid and found that another liter of water was required to dissolve the remainder. That solution is quite a potent developer, but not as strong as what I decanted.

The CRC Handbook shows K2SO3.2H2O, with solubility in cold water of 1000 g/l. My 320 g/l certainly should have dissolved, even if I had the anhydrous, in the hot water.