Yellow tips on newer leaves. Nutrient burn or something else?

I think I found the issue! Thanks to @MrBlocc420 and @LionGrow for mentioning to check the calibration of my PH pen. Somewhere along the way it got out of whack and was reading about .4 to .5 higher than actual. So when refilling my res with 5.5-5.6 water it was actually more like 5.0 to 5.1. Not ruing out other issues at this point, but I think we have a winner. I re-calibrated, flushed and changed out the res. Hopefully things will clear up! Thanks all.

I think it's light burn mate. Back of the light a bit of you can, has happened to me before....

Thanks for all the feedback so far! I am growing in Coco running with Blumat auto watering system. I had had some high PH issues earlier in the grow due to growth in the reservoir. I corrected that (Bleach and H2O2) and have been monitoring at the res daily for the past two weeks. I have kept the res between 5.5 and 6.5. However, one thing I haven't been able to solve yet is that when I bleed out the water lines The PH is usually .2 to .3 higher than the res! But after bleeding through about 250 ml, it is back to what it is in the res. My thought is that the PH of the water sitting in the drip lines is rising over time (like 24 hours) but not sure why. Maybe growth in the lines? I am using supposedly light proof tubing. My action plan right now is to mix up some 1/4 nutes (from 1/2 currently) and flush both plants. Will then flush out the drip tubing with H202 in case there is still residual growth there and change out the reservoir with 1/4 nute solution.

Pull back on the nuits, correct your pH

I agree with MrBlocc420. check your PH and make sure pen is calibrated thats caught me out in the past. If the leaves start to darken with purple tint on top leaves near light defo will be copper. If the tips turn brown and dry then might be nutes. But because you had some PH issues. I would do what MrBlocc420 says and check PH run off and check pen

Are you using coco ? I had something similar with tips whilst in flower and realised it was drying out quicker so started feeding in lights on and then light water before lights off. I also added some calmag as my coco was pre-buffered so didnt need in early weeks seemed to resolve it