Ec going up during flush

No, the thing is, without getting to deep into mineralogy and crystallization, and trying to keep it clear and simple, the Nitrogen N turns into salt (NaCl) by crystallization. Your EC meter only gives you the amount of NaCl or KCl (depending on the the scale you have set on the EC meter), but not the amount of N before its crystallization. You might have give them some heavy feedings and too much N over week 4 of flowering and during the stretch. At the end of stretch the roots only process P and K leaving the N on the side and running from its own stock filled in the leaves. So what happened, there was still some N left in your soil and it has turned into NaCl, this is why your EC level is raising again. This is why it’s always better to do minimum a 2 weeks flush, to drain out all the minerals !