Seedling Day 10, and something is going wrong ?

Doing a DWC, grow pod bottom at water level directly above bubbler, Light 18 on, 6 off (Kind LED), 460 mmol, chamber temps & RH = 82F 60%RH w/light, 74F 75%RH light off, water temp 69F, pH 5.7 - 6.0, TDS 335 Seedling has sprouted third set of true leaves, ten days in chamber. pictures show two days ago, then today. Had been dropper feeding from the reservoir below the plant. resv had 25% rec amount of boost & grow. When true leaf set one started turning yellow (day , I lowered the plant about an inch into the reservoir for 12 LED light hours(day 9). Back above water level today.. see attached pictures.

I'm just guessing here but your feed might be a bit too hot...
I only measure in EC (ms/cm), and if i've got the conversion correct, that gives you an EC of 0.5, which whilst not crazy, might be a bit too much for such small plants in DWC.

What are you feeding them on?

Also I'm not an expert, just my observation based on my limited experience with DWC.

I had 25% of the recommended General Hydroponic BOOST & GROW. I dumped the water yesterday, and refilled with RO water, then reduced the pH with GH Ph down (buffered phosphoric acid) No nutrients added yet. TDS =77, EC 152, ORP 293. pH = 5.92 / water temp 71 F.

Plant holder cup/pod is in the bubbling aerated water about 1", leaving 2" above the water level. Plant is still drooping badly, same leaf set yellow. 2nd, 3rd, and now a fourth leaf set are green but droopy. Still totally puzzled as to why the plant took this turn downward in it's second week in the chamber. Don't know if it needs more/less food ?, water ?, oxygen ?, light ?

Unless anyone else chimes in, id back off on the feed, you say you were using 25%, back it off to about 12% and see how you go.

Once the roots are touching the water, you don't need the cups/net pots under water, the roots will grow and find the water on thier own - thay may have too much water if the whole of the root pots are submerged.

In terms of oxygenating the water/nute solution, what are yu using, air stones/ are you getting a decent amount of bubbles?

Right now I have zero feed, seedling pot just above the water level, so long tap root is in the water, and all the other roots have sprung out of the little cup, but are above the bubbling water. Bubble picture attached. Have to figure out how to send a clip of the bubbling action

Looks overwatered

Seems like there is not enough root structure actually getting into the water in my opinion. However, I see you posted that the water is just below the root plug, so it could be that you need more space between the water and root plug so that it doesn't over-water it accidentally.

pic shows root structure today.. long root has always touched the water.. the others are probably getting splashed by the bubbles.