Not sure about the soil in the middle being hard, but thats where the roots form at first so it could be down to that.
Oh, and leaves fall off/turn yellow/die during flowering, its nothing to be worried about... as long as it starts from the bottom up, that is. If its only got a few weeks left until harvest, at least youve got the flushing out the way so that when shes dried ittl taste a lot better. When I buy Herb that hasnt been flushed properly, I always get a headache, whether its schwagg, mids or 'dank'.