Nutrient problem??? Excess or deficiency?

Forgot to mention… before flowering, the leaves developed a dark purple underside. Since these are Northern Lights, I thought that was a desired attribute and thought nothing more of it. Now I am wondering if this was a precursor to the leaf burn?!?

Nitrogen toxicity. Plain water, nothing else for next two weeks, I agree. Forget molasses, it does not do anything much. Far better to give amino/fulvic/humic acids, or all 3, they will actually do something tangible and beneficial for your plants. Wait 2 weeks though. Some seaweed tonic given at half strength may be beneficial after the first week of plain water and before resuming feeding in 2 weeks time, at half the strength you were using before. Hope this helps, Organoman.

My guess would be potassium but have you checked the run off water ec pH to see if they are in nutrient lock out? It might be worth it before you add anything else. I am no expert but that is what I would do 🙏

It looks like a pottasium deficiency to me. Considering that it is affecting the whole plant top to bottom and pottasium is a mobile nutrient. There could be various reasons for this. I would suggest adding some sort of liquid nutrient for this but with pottasium, an excess of the nutrient can cause it to lockout. You'll need to look through the history of your feedings to figure that part out.