The recommended DLI for pomegranate grown indoors (in a greenhouse or under lights) isn't information we can find, but if we extrapolate from the needs of greenhouse-grown tomatoes and peppers (some of the most common fruiting plants grown in greenhouses, with ideal DLIs of around 20+) and the DLI of natural sunlight in outdoor Maryland gardens over the course of the year, the seedlings you're growing might need much more light. A PPFD reading of 200-400 sounds a bit low for a sun-loving species like pomegranate, but without a diffuser, those readings might be skewed. (We don't know by how much...phone apps to measure PAR light have not been formally tested by Extension. Informally, we've found that sometimes they can match-up fairly well with a PAR meter, and sometimes not, in both situations without a diffuser over the phone's lens.)

What is the residue visible on the pot and leaf surfaces? It looks a bit like the material used to distribute beneficial predatory mites from biocontrol companies, but we're not sure. You mention applying a weaker fertilizer dosing, but not how often it's applied. Seedlings at this stage of growth probably don't need much in the way of supplemental nutrients yet. Too high of a nutrient content in the potting mix can "burn" roots, interfering with moisture uptake.

We can't rule out root rot, though it's hard to tell what is the main factor in plant decline. If you pull any dead plants out of their pots, investigate the roots: if they are somewhat brown and mushy, or pull apart easily so that the outer sheath pulls off of a stringy inner core, then that suggests root rot. (Roots dead from other causes tend to snap more cleanly in half, and take more effort to break.) The soil shouldn't stay consistently damp, especially once seeds have germinated and grown a few leaves; while they are germinating, a humidity dome over the tray of pots can help retain humidity without having to water often. By the time seedlings have grown large enough to outgrow the dome height (there are humidity domes taller than the typical 2" or so, around 7" high), they can be weaned off of a consistent moisture level.

Few Maryland gardeners grow pomegranate (especially from seed), so we do not have information on the nuances of its cultivation, particularly starting them indoors. We have seen recommendations to start pomegranate seeds inside around 6-8 weeks before the last frost date, which would tend to put the sowing time around early March, so since these were started very early, that may add to the challenge of keeping the seedlings vigorous until they can go outside in May. In general, leaf drop might be due to insufficient light (though leaves tend to yellow first, not fall while still green), watering irregularities (when they do need water, make sure the pot is soaked thoroughly so the lower roots aren't drying out), pests (spider mites, thrips), or infections like powdery mildew. We don't see signs of the latter two issues, but it's hard to see enough detail in the pictured foliage. Sometimes ethylene (like from a leaking gas appliance, etc.) can cause plants to shed leaves and petals (in bouquets, for example), but if that were the case here, we'd expect other plants to be having similar symptoms.

For now, just try to give them a bit more light, if possible (incrementally lowering the fixture might help, though it will be harder to get the tallest seedlings to fit), and let the soil become somewhat dry to the touch around an inch deep (perhaps less) into the pot before watering. Inspect the upper and lower surfaces of the leaves for pests, and consider pausing fertilization or spacing-out how often you fertilize so the potting mix gets more of a "flushing" with plain water to help leach out any excesses.

Miri