This is a calcium, NOT a potassium deficiency... and, while it is common, it is NOT normal and IS a problem! In flower, plants need more calcium so introduce some Calmag to your nutes but only once a week... you won't see any improvement in the leaves that are already affected but the problem should not spread....
Good luck!
It's a P deficiency but DON'T WORRY! It's perfect and normal at this stage of flowering, keep flushing for one week and the you can harvest :) if you want you can cut off the died leaves, to avoid mold, but it's not necessary