I'm afraid I don't quite understand what you mean when you say everything is "all good" as far a nute levels in your soil. There shouldn't be ANY nutes in your soil so my very first guess would be to change your soil. Someone above suggested overnute and or/too hot fertilizer and I agree. And I assume that again....by "fine" you mean that both your source water and your soil runoff are between 62-6.8. If anything is out of those ranges I would NOT consider it "fine."

I like to start my seedlings in rapid rooters, then once they reach the size of the one in your photo (a few days) I transplant them into Fox Farms Happy Frog in 4 inch pots for about 10-14 days. And mine do fine that way so I offer you the same option.