Any idea why some of the top leaves of my plant are fading?

The lower leaves going yellow are a normal process, whereby your plant will re-use stored elements from within her older leaves to use for new growth during flowering, as it is more efficient to do so than to make new sugars, amino acids, carbs etc AND try to grow flowers all at the same time and is one reason why defoliation is such a counter productive technique. I also believe your plant is suffering immensely from the stress being caused by continuous lighting. No plant has ever evolved to grow under 24 hour light and cannabis can only grow so much during any 24 hour period, and this growth tops out at about 18 hours, meaning giving your cannabis plants any more than 18 hours of light is basically a waste of electricity. Plants, including cannabis, do actually need to "sleep" and undertake certain metabolic processes they can not do during the lights on/day time. By subjecting your plant to an endless day, and for so long already, I would say the leaves are indicating a very over tired and stressed plant that has been wondering for many weeks already, when she is going to be able to have a rest from all the light. Continuing with 24 hours of light could lead to the plant "collapsing" and with an incomplete terpene and cannabinoid profile due to not having her "sleep" time to restore her health and repair from the days growing. Going back to 18/6 will not slow growth, as this is the optimum day length for cannabis, derived from 80+ years of scientific research and is not a random number. In fact, I would not be surprised if her health did not dramatically improve and her leaves returned to normal, once she is given a chance to sleep, just like she has evolved to do over Millenia. Continuing on by thrashing her with maximum lighting will just cause something to give, no machine or living being can run at maximum speed forever, something will break and it could well be your plants flowers. Go to 18/6 and give her the sleep she is designed to have, and she will no doubt be a much happier and healthier plant, with much more complete flowers and cannabinoids. Continuing with 24 hour endless light and without a natural circadian rhythm, and she will be unable to produce her best quality flowers. She needs "sleep" to be able to function normally. People who advocate for 24 hour light, in my opinion, do not understand the basic principles of nature or basic plant biology either. 24 hour light is completely unnatural and again, in my opinion, basically plant torture. Nothing can perform to its best ability without having a rest every so often. For cannabis, this peak performance formula is 18 hours of light time and 6 hours of dark time, derived at from hundreds if not thousands of scientific experiments over the last 80+ years. Hope this helps,...... Organoman.

Looks light light stress. PK toxicity tends to be more uniform whereas only your tops and oldest foliage seem affected.