Glad to have ran across this. I too have had bad issues this year in my guerilla grow with purple tips/leaves on the new growth. Mine was worse in June and July and seem to be coming out of it. I started raising my phosphorus and cutting back on the K. I have noticed when I overuse seaweed and kelp products I get this symptom and have learned that too much blocks out the uptake of phosphorus.
I'm also a little stuck on it quiet possibly being a disease as well, and I have never had an issue with it until this current year/season.
I am fixing to hit mine with a fungicide called FLINT by Bayer. Purchased it last year to use against budrot and it worked. Stops it in its tracks and has a low toxicity level, have never in 17 years had a product work so well for botrytis. Its quiet expensive as I gave over 200 bucks for a very small container of it but it was worth a thousand dollars or more in the end and a little goes a hell of a long way. I need to be starting it soon anyway to keep down chances of a disease outbreak starting, as prevention.
Lets keep each other updated with this, seems like a serious issue that we need to be taking notes on and experimenting with. The affected growth I notice is stunted as well. I know with low P levels or deficiency comes with red/purple stems as well which mine are also exhibiting. It is also more on one particular strain (1969 Uncle Festers Skunk#18 ) than it is on others.