Help! Dendrobium leaves on one cane turning yellow

[Lady Oscar]

At the beginning of July I bought a Lowe's rescue Dendrobium aridang green (Lowe's rescue - Dendrobium aridang green). It had small black spots on the leaves on one cane, but not the others.

The plant has been looking happy since then, and the spots have not spread. It's currently growing two new canes, and there are a bunch of new roots showing at the top of the media.

However, this morning I saw that the leaves on the cane that had the black spots have abruptly turned yellow. This plant sits on the end table next to my elbow in my usual living room seat so I look at it many times a day, and it was definitely fine yesterday.

I thought a bit of sunburn, since this plant is the closest to a window that I have, but if anything it's been more overcast lately.

The last time I watered it was three days ago, using fertilizer at 1/4 strength as usual.

The leaves on the other canes, new and old, all look perfectly fine and normal.

Does anyone have any idea what the problem could be, or advice on what I should do? I've become very fond of this plant, and I'm horrified that it's suddenly unhappy!