That happens to some of my seedlings sometimes too. Its nothing to stress over. Its growing healthy new leaves just fine. Plain water and light and its good. Start nutes again in about 10 days when its a little bigger.
Rule of thumb is dont start feeding until the cotydelons (those little round leaves all the way at the base) turn brown and wither away. The cotydelons are made to feed the plant until it develops roots.
This is a picture of mine that did the exact same thing at the same point in its life as your plant did and it bounced back just fine and is strong. Mine did it after I put it outside in the sun where it got sun all day.
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