Oh your poor seedlings. I would be highly suspicious of that potting mix. It is obviously something that all of the seedlings have in common. I know that you have used this brand before but not this BAG. Possibly something is wrong with the mix since every plant seems to be affected. You could not have damaged every single plant during transplant and if it were aphids, I would expect to see them if they were causing that much damage and I would also expect to see them on the smaller seedlings.
If it were me I would take a couple of little seedlings that are not affected and plant one in that Miracle Gro mix and another in a new mix and compare results.
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