Streching

Increese the lights intensity. going from the same intensity at 18/6 to 12/12 is a new loss in overall light. Look up DLI to get a better understanding of this. With my lights. My Veg/Flower both hit a DLI of 37 Veg at 40% intensity for 16/8 at 30" hang height. Bloom at 60% intensity for 11/13 at 30" hang height. More light causes them to keep the node spacing tight. However genetics also has a role to play in that. Some plants are just long boys and they need help with it. Ooptions are bending them or snapping their main tips SCROG/Super cropping.

Reduce the amount of red and far red and give them more blue light. More light in general will help. I think more nitrogen will make them stretch more too. Apparently the reason plants stretch under shade from other plants is because the shade gives off more of a red wavelength of light that cues them to stretch to get to better light. Genetic will play a role as well. some plants are just going to stay squat no mater what. Keeping your temp differental closer togeather apparently helps strech too.

At the end of the day this is gonna be hormonal and strain dependent, but you can try your best to fight it. I personally love large internodal spacing..all flowering sites get ample light that way. I also use it to train into my canopy. If I were trying to beat it though, your best chance is having apical dominance thwarted for the immediate time period prior to flipping and to keep doing LST to ensure this. Recall that 50-150% of the height of the plant is the old rule of thumb, with 50% growth during stretch in height being just the norm back then for the types of lights