Bulbophyllum Sumatranum Leaf Discoloration

[LilyFlower6]

I've recently purchased a Bulb. Sumatranum. It looks quite healthy though it needs a repot since the new pseudo-bulbs are growing outside of the pot.

However, I'm having an issue with one of the leaves. I know bulbo leaves turn yellow as they mature and grow old. But the one in the picture has turned brown. Not sure whether that's an infection. The leaf right underneath looks healthy. Any ideas what it is and why it's happening?
Thanks

[estación seca]

Could be too much light. But the commonest cause of leaf browning in Bulbos is inadequate water. They prefer to stay very wet.