Yes agreed hence the flushing, its possible soil was overfed and PH is bad
I reuse soil in my fruit and veg plants (up to 5 years) all the time, never been an issue
I flush/wet heavily once a week and drip feed 2 mins a day and have no issues with PH
Which is why Im suggesting a flush of 10%
I would say frequent flushing is just as good, I just like to keep it simple
There is a chance this is caused by overwatering which flushing will not help