I vote for overwatering as well.
Be careful measuring runoff ppm/ec. As the water moves through the medium and container it will pick up residue and salts left behind and will increase the measurement substantially. This is why I don’t like to measure runoff ppm. If you have a container full of nutrient rich growing medium like soil and the container is 6” tall you’ll get a vastly different reading than the same soil in a container that’s 10” tall due to the leeching of salts and buildup. A slurry test is preferred when possible for ppm/ec. But that’s my opinion.