One plant (the one on the right) is going yellow, ...

to be fair that is not the signs of P or K deficiency, either. Since it seems to originate at the veins and kind of odd symptoms, a root zone issue is not out of the question. Between the two options i'd lean root zone since i see no symptoms of p or k deficiency. if soilless or hydro and fertilize every time, if ppms of these are 50-60 for P and 180-200 for K, you probably don't have a deficiency in either. In soil this would be more of a guessing game, but with soilles hydro, these levels are rock solid for all but the most esoteric needs of an oddball plant. All these 'fancy' nutrient systems with 4-5-6 products or more to use at this moment or that is just a bunch of nonsense to feed soemone's OCD or ego. The plant simply needs a well-balanced diet and that shifts slightly in flower. Studies show dumping a shit load of extra p or k doesn't do anything for yield. there's "enough" then there is waste above that provides no return or even can damage the plant in extreme cases of overfeeding. Take notes and know what you add in total as well as individual concnetrations of each nute, if in soilless/hydro. It makes such decisions much easier. adjust future strategies to avoid the problem next time with a proper history of feeding and more consistent provision of necessary nutrients. in soil, there are so many unknowns, but this can help a little bit even so. that's not a knock about soil.. just facts. still plenty of good reasons to use soil if it is your preference. the same needs are met, but the delivery method is quite different due to nature of typical soil nutes and amendments that come packed into soil that can last for months. This is why in soil you may need to add different stuff at different times due to it being used up in the soil. whereas with soilless, you provide 100% of needs every single irrigation. this makes diagnosing much easier.

Starving for potassium with a developing calcium deficiency. "Flushing" has probably made things worse, not better. Flushing is an emergency intervention in case of severe overdose, not a treatment for deficiencies. Add a P/K booster to your line up or increase the dosage if you are already using one. Same goes for the cal/mag supplement. Leaves will take 2-3 weeks to show improvement. All plants are individuals and have individual requirements. Not sure why anyone would think this is "roor rot" though.