late start

The tip burn is nearly always caused by excessive Potassium, and it's extremely common in late flower due to build-up in the medium. Dial back a little on the Potassium in the feed, and/or flush that new feed through the medium till the run-off and feed measurements are the same. Potassium ions are used to transport ALL other ions within the plant; they are "Ion Pumps", and too much pump pressure causes damage that first shows at the tips. ;) The class of PK boosters sold to home-growers are mostly a marketing scam, praying on ignorance, and for the most part lower yield through often invisible damage. Nice plants BTW, after a trim you won't be able to tell this happened.