I use HP all the time and almost every strain gets the very tips going brown. Even in DWC they did that so it's a fairly normal thing.
If the tips along the edges of the leaves do it and the edges turn up then it's likely overloaded with salts and needs a minor flush to lower the levels.
900 is pretty high so I'd back off now as their feeding slows down a lot after the stretch which should be in a week for you if those are 8-9 week plants.
What type of nutes are you using? Water? Tap or RO? 1ml/L of each of my 3-part gives me about 300ppm and I rarely feed more than 2ml/L every 2nd watering about 4 or 5 days apart when the pots are dry enough.
I think a couple waterings with plain water or a little bit of bud booster will be enough to keep the burn from progressing if it is burn at all.