Please help identify the cause, interveinal chlorosis since 2 weeks old, no necrosis so far. These two plants are 26 days old. Do they look stunned?

I would try and water the entire pot vs just parts of it. can help avoid dry spots.

Yes, magnesium deficiency.

I had this happen to my Pink Mist Autoflower. I turned down the lights, added a bunch more veg nutes, and now she's looking much greener 🤞💚✌️

Can't see a Mg-deficiency until 30+ days after it starts, so you can rule that out. Does not look like Fe progression (more of a patiole-out progression). Maybe Zn? A trace element issue is rare. if in soil, should have it in the soil product and if in soilless/hydro one part of fertilizer setup should contain the trace elements. So, just ensure that is the case. After that you just need to make sure pH is not the cause or some sort of whacky imbalance - is something sky high in concentration? I don't think that's likely as there'd be more than one issue present more times than not in that context. Looks like you have some variegation in those intial few sets of leaves, and may be related to that which is harmless. New growth looks like a solid green. I would continue to take a wait-and-see approach.