DSP Genetics | Double Berry Cookies

You either have pests, or two separate issues. The brown tips are likely nute burn or bad pH deficiencies. You already flushed, start reintroducing nutes. If it stops spreading, you're good. The brown tips that you already have are there to stay unfortunately, but it shouldn't hurt your end product. Heat or light stress is also possible, but the least likely with your others thriving so much in my opinion. Of course different strains behave differently. Good luck, keep a close eye out for pests, and don't overwater.