Dried cotyledons

Does not matter................cotyledons do not serve any purpose after germination.

Is it better if the cotys remain healthy in the early stages, sure, but cotys aren't too important. Not enough info, but could be hot soil.. or if it's "soilless" its fine to give light fertilizer from the start. Though at this stage, the contents of the seed should still be enough to power growth (~10 days or so). It'll slowly need more light. As you see it stretch, increase light, but it should continue to grow. Healthy stretch is okay. Some spacing between new nodes should progress but not be weak etc... there's a happy middle ground and how it grows is the guide. Everyone's local variables are slgihtly different, and that can greatly impact light needs compared to other gardens. one-size-fits-all only works for gardens with very similar and tightly controlled enviornments, whic may or may not be the case. The resulting pattern of growth (morphology) is the end result of those variables, therfore a good guide. You can take notes of how you did it this time, assess results and adjust for a better result next time... just how it goes. Like i said, variables are often quite a bit different for your garden and the next, unless both are very tightly controlled in similar ways -- highly unlikely. Seasons may make a difference too, so be ready to adjust on the fly throughout the year. You shouldn't have to water much early on (referencing time between irrigations). It barely has any roots. So, adhere to a good wet-dry cycle at this point. Unlike a very recently germinated sprout, this has roots that go to the bottom of the cup by now. You can stick to more orthodox irrigation practices at this point. So, allow the surface to start to dry before re-irrigation. This will keep the rootzone healthy - a good wet-dry cycle promotes root growth and ensures they drive downward, not upward. Learning loss of weight is an even better trigger for irrigation than how dry the surface is. Always fully saturate... partial watering is an absolute no-no. The only time it can be justified is when someone puts a very tiny plant into a very large medium.

Try as @Growmebig pointed, but this could also just be PH irregularities or just your plant pheno not struggling to your environment. I took a look and your other grows seemed to suffer a bit like that so watch for: * Nice and regular PH * Not much light * High RH * Watch you nutrients lvl, maybe tune it down

Shoot for 75 degrees, 80% Humidity, and 100 PPFD light. What are your Numbers? by looking, I would Say you blasted it with Light, too soon.