In hydro problem are fixed in a matter of days not weeks. Watch them closer then that. How close if the light? The temperature you are mesuring may be under the light but what is the temperature of the leaf?

If the light is to close then yes you can even burn them with that temperature range. The internal temperature of the leaf structure may be much higher then what you are reading in the air.

If you are that far into flower then I would guess it's more of a nutrient lack then nutrient burn from to much/ 800-1000 ppm is for veg cycle not flower cycle nutrients. That late into flower you should be more P & K then N at this point and your buds will be small and thinn if you don't give them the right nutrients at the proper time they need it.

That is a few things to think about.