After beeing red, let's turn purple

Your plants are looking so nice!! If the leaf got a little bit of water, or too much humidity on it i.e from sticking with another leaf, then the water dried from the light you will see spots like this. I wouldn't be too worried about this! Awesome job with your techniques, we are all excited about the evolution you have here! 😍😍

It looks like them are the lower leaves on the plant. Where they don't get as much light. That can cause your leaves to show deficiency. Buy my feeling is you may be getting a problem. I would increase your ph a fraction to make sure your plants absorbing as much a range of nutrients as possible. Good luck

15" away on a 150w led, I would find it harder to cause light burn at that level. But I could be wrong. the PPFD map on amazon is showing a low of 400 and max of 663 dead centre for 15" height in a 2x2 grow tent. and that's about the size of yours. https://www.amazon.ca/MARS-HYDRO-Lights-Lightweight-Spectrum/dp/B07QBZ3XFG?th=1 The dry tips and curling up do look like nutrient burn. It could be happening at the top of the plant first because that's where is repatriating more and taking in the most energy. This link is a goof little read and helping to identify some things. https://www.growweedeasy.com/why-are-leaf-tips-brown-burnt-or-yellow As for your weeks leading up to flowering. your where at a steady PH of 6 the it jumped to 6.5 just on the diary published number, but in the then back down to 6.2-6.1 for the following weeks.diary entry your PH is jumping all over, one water its 5.5ph going in runoff is 6.4-6.5 going out, the next watering it 6.4PH and no runoff. if I'm reading it correctly. In coir and using liquid nutes I always thought it needed a PH range of 5.5-6.2 I cant help but think this is a larger issues caused by PH fluctuations in the medium. But the other plant is looking proper suggesting it could be more light as the problematic plant is higher up. Its a bit beyond me. Some of the ppl here who are more skilled in liquid nutes should be able to provide a better reason. Best of luck.