Just went through the same thing w/ some BLZ bud seedlings. At 5-10 days they developed yellow central vein. Battling some high temps 95+ F daytime, and 85-90 F eve temps, so I re-calibrated PH instrument. Sure enough, ph too high. Running hydro, so I adjusted ph and all new growth was fine.
PH is key, so check often, especially in hydro, and when temps are running high.
yeah my ph is around 5.7 to 5.9 all the time and i have 2 oscillating fans so it was strange that i had this problem but i think it was a little deficiency though due to the root problem i had earlier but thats almost gone now and both plants look a lot healthier and one of them has really taken up the vegging spirit.