My S.Japonica I bought from Andy a month ago started yellowing leaves rapidly. I cut the dying leave anyway to check if there is rot in the crown, and I do not really see any sign bacteria infection in the stem or crown. Whatever it is, I hope it wont infect the neighboring plant. Ironically yet, one of its root is still growing.
On the third pict, is it pupping another growth?
The growth with the yellow growth looks dead, the crown looks dried up, but since it has a good root, it still may pop out a new growth. No more cutting... But the green growth (on the left side of the photo) looks fine. And the green growth may, indeed, be producing a new basal growth. These do produce basal growths, forming a clump rather than a tall plant. So the old growths may die back, but this growth pattern is normal as long as there are also new growths.
In southern California, this species really doesn't need a greenhouse. Mounted, it does need daily watering. (I grow it outside, I strongly suspect that Andy grew it in a shade house rather than greenhouse, but his nursery is humid, he waters A LOT.)
I think that yellow leaf was just an old dying growth.. the crown doesn't look great either. I would not have cut the leaf, much better to just let nature take is course. But the new growth looks fine.