Oh man, I got reprimanded by soil growers in here for giving advice on soil while I only grow hydroponics
and yes, they have a point.
I don't know about Miracle Gro soil and their slow release of nutrients (or as someone in here said, they release nutrients when you add water).
All I can do is share an old grow photo of mine having these symptoms but they were more severe and for me, it turned out to be a nutes deficiency. It started with one lower leaf, then the second one and it just kept progressing. Here's is the photo (note this plant looks strange because I topped it very early):
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It may be a PH issue for you that's locking out some element uptake.