Brassia with rotting pseudobulb?

[VeoVerde]

Hi guys,

I was gifted a very small Brassia Rising Star a month or so ago. Either when I recieved it or shortly after (I thought I inspected well but now not sure), the smallest pseudobulb had begun to discolour. Now the leaf on top seems to be dead.

I'm guessing that being the smallest, it's the oldest bulb, but I'm not sure? If it is, I suppose the loss of it wouldn't be catastrophic, but any ideas on what it is, whether it's likely to spread to the healthy pseudobulbs, and what I can do to help it would be very welcome!

Attached are photos of the browning progress (worst is the most recent). We're in winter here in Sydney, Australia, but it hasn't been so cold. the plant receives bright morning light through a window on the opposite side of the room, and medium shade from about 10:30am onwards. It's in a tiny pot, so I water maybe twice a week, when I feel the pot lighter (and dryer).

Thanks for any thoughts!!