No cause for worry, first of all your plant is the picture of health itself, as you can see ! so knowing this, it's not just the stretch that happens, and when it does it's not like an elastic that gets thinner, the stems thicken too, the roots dig in more, the older leafs will get darker and the newer growth is always thinner and lighter than older growth. The only real issue is see is some Magnesium deficiency on one of the plants, you can tell from the yellowing in between the veins of the leafs and the discoloration/burn on the sides of the leafs affecting older growth first. Up your cal mag and make a cal mag foliar spray, keep these in veg an extra 10 days and they'll all enter flower in 10/10 shape. Hope this helps ! 🚀