I saw it on one of my camellias last year and it was iron deficiency but it could also be nitrogen deficiency. Iron deficiency starts as patchy foliage and yellowing between leaf veins with darker leaf veins (the top foliage affected the most). Nitrogen deficiency shows up as yellowing leaves but starts from older leaves. Might be worth checking the soil pH or adding Epsom salts or chelated iron tonic. The problem though is that one half (or one plant seems fine) seems fine. Odd.