Answer: It sounds to me like they may be receiving too much water. When you transplant from small containers filled with a light seedstarting mix to larger containers with heavier potting soil, it's easy to overwater. Often, damage to new growth indicates overwatering, so that's my first guess. Let the soil dry out somewhat between waterings, make sure they are getting plenty of light, and see if that helps.
If you mixed the fertilizer according to label directions, it should not burn the roots. Sometimes, over time, fertilizer salts will build up in the soil and damage plants. However, it's unlikely that this would happen in just a few weeks. If you feel you may have overfertilized, you can try leaching the soil by flushing it with lots of water, then letting it drain thoroughly. However, this won't help if the problem is overwatering, so do this only if you have reason to suspect a fertilizer problem.
Hopefully, you're plants will make a full recovery!
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