Browning tips usually mean overfeeding, or there is a nutrient buildup from not proper drainage.

Nute buildup happens when proper drainage isn't there. Like, when you water, and no water comes out of the bottom of the pot, that will cause a buildup of nutes for example.

You said heat isn't the problem, and by the temps you gave it shouldn't be, unless the top is too close to the lights, or are touching the lights, that will happen.

I'd ease up on the nutes for the next few waterings and see what happens, because by the sounds of it, it is getting too much fert.