OK I think I've got it figured out.
While I eliminated humidity and temp early on as a cause, that may have been me jumping the gun a bit.
I've got the ppm's and the pH of the tipped out water, tested a day after sitting, both stable. Ppm's around 700-800 and pH 5.9-6.1 across randomly tested plants.
But I'm still seeing shorter, slightly pale plants with purple stalks......
1) I noticed that my 4x4 tent seemed more humid than my InkBird humidity sensor was saying the tent was at. So I threw in a couple other sensors of various brands and it was reading low by about 10 percentage points. So my 65% humidity cap was actually letting the tent hover in the 75% range. When the temps are about 23.5-24.5°C, that's a little too much humidity for older plants. That would make my VPD plummet and slow nutrient uptake.
So I've adjusted my InkBird RH sensor to match the other sensors.
This was the primary issue at first, but solving this wasn't the whole solution (see next points)..............
2) This Melon Farm 4x4 tent is a piss poor design. It only has a single rectangular mesh breather port down low. No 6" round intake so I can't add an intake booster, and no extra ports down low for passing piping or cords thru. And the location of the breather vent in comparison to the ceiling exhaust ports meant the air wasn't being drawn up and out properly or circulating the tent properly. It was siphoning the heat out but leaving humidity down low at the plants.....but the constant running fans was also possibly keeping the roots a little cool....maybe as well.
Who TF designs a grow tent with no lower ports and a single breather vent?
$&#@!? morons, that's who. Nevermind it doesn't have side doors either. Have fun getting at your back plants......Don't buy Melon Farm tents!
3) this 4x4 veg tent is the only tent in the whole bunch that doesn't have active data logging or any kind of controls I can interact with. It uses just one of those cheap sensors that you place in a convenient location to monitor, and all the controls are all analog sensors (a wall plug temperature sensor, and the InkBird RH sensor) that run on their own........
This readout is all I have, but the readout doesn't update as frequently as my other sensors (the others update once a minute) so I was unable to pick up patterns, only able to see what the readout said itermittently. It doesn't, for example, allow for me to see that the exhaust has been running for hours instead of a nice cyclical pattern.
Basically a perfect storm over the last few weeks......
So I've taken a dehumidifier and added it to that room. This should suck the moisture out of the air while also driving up the temps a little. This should raise my VPD and increase nutrient uptake. Combined with the balanced out ppm's and pH, I should hopefully start seeing even further improvements.
I am buying another 4x4 tent, moving everything from this $&#@& Melon Farm, and making this MF (suitable initials for this POS tent) back into strictly a drying tent. It's absolutely useless for growing in, especially for someone with a bad back, not having side windows.
I'll be going into the larger flowering tent soon anyways, and up-potting, so hopefully this is all behind me soon.