Please Help?

Without a diary and/or a lot more information and without knowing exactly what you may be doing or not doing, giving any sort of meaningful answer is almost impossible...........start a diary, especially if you think you will have more questions in the future. Best guess....since it is affecting the new growth, it is most likely a deficiency of minor nutrients (trace elements). Easily solved with a dose or three of a soluble trace element/trace mineral mix to the root zone. This product should be available from any decent garden centre. Do not use foliar sprays. I would also advise against ant sort of unecessary defoliation,, your plant needs those leaves to make the flowers grow........buds do not need direct acccess to light to grow, but your plants does need those leaves to make the energy for growth. Just remember........... Less leaves = less energy being made = less potential growth.

What 01101… said. Id additionally remove the fan leavs that are covering shoots that will produce later on. Defoliation leads to more nutrient consumption, maybe you want to try out this one: https://www.growweedeasy.com/nebulas-super-simple-3-step-defoliation-tutorial No need for a flush if you think about it. Make use of the extra nutes in your growing media

You also increase light intensity while reducing to 12hours of light? Possibly overdid it. Should be roughly 150% "more" if you went from 18h to 12h. Did you increase it more? (assumes you dialed in giving near-max DLI for your local environment in vege). other than that, possible an increase in p/k of bloom nutes can cause light green newer growth - initial stages of a buildup/toxicity. I'd let this progress a bit more. More than one possibility exists unless you can effective eliminate them through cross-referencing information not given in this question. diagnosing purely from leaf symptoms amounts to a guess. The cause and effect is not descrete.