Superthrive seed soaking?

I know not everyone agrees with pre-soaking seeds, but... for those who do, has anyone ever encountered an issue with soaking in Superthrive?

I have a struggling bunch of seedlings, and I'm running through every possible cause of their dilemma. The only think I did drastically different this time was soaking in Superthrive solution (1/4 tsp/qt).

They grew fine for about 2 weeks, and right about the time they should have been hitting veg and growing more quickly, they slowed down and the lower leaves began to yellow. All the centers are green and healthy-looking, but they've essentially stopped growing, and the yellow is now turning to brown. I've never seen seedlings just sit there, day after day, with no physical growth, just slowly browning out older leaves. Stems are straight and strong, they're not wimping out. They've been like this for about 14 days so far.

I grow in composted cow manure, but 3 of the 8 were in rockwool, and same thing there. Tap water that is 6.5 pH, dechlorinated. No nutes til about a week in, then 1/4 Miracle Grow. Once the issue developed, I went back to straight tap water.

But this is fairly baffling. Have seen them thrive and have seen them die, but these are just in some kind of permanent limbo, where nothing happens.

Well, a couple of things. I haven't done the superthrive soak, but 1/4 tsp in a quart sounds pretty heavy duty as you're only suppose to use one drop per gallon. But if your method seems to work fine, then stick with it.
If you're plants are growing and turning yellow, I doubt if it's the superthrive but instead the nutes or lack of, that is the problem. MG has quite a bit to it, but it is also hot stuff. If I were you I'd change my fert use to something that a grow shop sells, even though it's more expensive. You could get an 8 ounce bottle of Dynagrow 7-9-5 and just follow the direction on the back. There are many too choose from; too many really. The powder ferts work good too, like happy frog or whitney farms. One cup of fert in a 5 gallon bucket will do. It sounds like you primarily have a fert problem so trying something else might be a good idea. Pics next time would be good. Composted manure might not be the best thing. Foxfarm or Roots or Promix or other decent grade of potting soil. It might be too late to fix your problem now so try taking cuttings and growing those and see what happens.