Looks like overwatering to me. The root zone is getting starved of oxygen.
I also don't know if I agree it needs a flush might get away with just water and no more nitrogen rich feeds for a few weeks.
Best to monitor the concentrations of what your feeding using a TDS pen. it reads thing in EC/PPM and that will be a good indacator if your over or under feeding the plants.
Good Luck!
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