What about the smalllest one the leaves are yellow between veins and slightly twisted after my research last night I came up with a magnesium def and acted on it since they were due for a watering with just under a 1/2 tsp/ gal of cal mag at 6.3 (my last watering was 6.7). Lmk if that diagnosis sounds correct. I know my pics are terrible and hard to see
Because these are bag seed, you can't necessarily expect uniformity between the plants even though they came from the same bud
It could be the plant in question is particularly sensitive to PH and had some lock out. You've done the right thing giving the low dose cal/mag adjusted to 6.2, hopefully this will sort the offender out.
Though we do have to consider that she just might be the throw back carrying the undesirable traits, hence why she is small. That's just a maybe though. It's harder to judge with untested stuff.
Give her a few days to adjust to the new feed and fingers crossed you'll see some good results.
You never know she might stay small but produce super dank buds. I've had this from tester seeds in the past.
Google 'the theory of infite dank' its a good read
You're doing well so far
keep it up