Bulbophyllum Elizabeth Ann Buckleberry leaves suddenly deteriorating
 

I got this Bulbophyllum Elizabeth Ann Buckleberry in December, and before very recently it looked happy and grew a new bulb. I had it in the coco fiber basket with sphagnum mixed with perlite and leca

About two weeks ago when watering I noticed some roots became mushy, so I took it out of the basket and discovered it lost half of its roots, they became mushy and velamen stripped from them.

I thought it was a root rot meaning I was able to overwater a Bulbophyllum (facepalm). So I removed old sphagnum (it was all dark green with algae), add some fresh sphagnum and reduced watering a bit.

Then a few days ago, leaves on the two seemingly the oldest bulbs turned half-yellow overnight, I though maybe reducing watering was a mistake, gave it a good soak and went back to my previous watering schedule (misting daily, good soak 2-3 times a week - I live in Barcelona and indoor humidity is pretty hight here, so it never dried up with this schedule).

But leaves continue deteriorating and in three days they became black and snapped out :(

I checked roots again and I saw that only the newest bulb has plump good looking roots now.

Clearly, something was wrong with either potting or watering, but I don't know what and what I should change..?
Also, are the bulbs that lost leaves dead? or they still could produce roots or new bulbs?

First two photos are taken on the day leaves yellowed, and others three days later

The pseudobulbs are somewhat shriveled, so it had not been getting enough water for some time. I don't think the problem with roots was too much water. This is a warm to hot growing plant. What are your temperatures?

Hmm, the pseudobulbs were always like this, here are some photos from the times when I just bought it Bulbophyllum Elizabeth Ann 'Buckleberry' - am I doing it right?

So, probably its issues began even before I purchased it?

My night temperature is usually 16-18 C in winter, and day temperature is 22-23 C in the room, and slightly higher at the plants shelves because of the warm from lights - I don't have good natural light, so I use only artificial light

It would do better at higher temperatures. I'm not sure what the problem is but I suspect that is the main issue.

I think you should keep it potted as it was, continue watering, try to raise the humidity if you can to compensate for poor roots, and hope for new roots with new growth.

It is very likely that there were problems with the plant before you got it. It tends to be pretty robust. I actually grow Elizabeth Ann on a mount outside (southern California, winter lows 4 deg C/40 deg F or so) It gets daily water, but dries out during the course of the day.. It's half Bulb rothschildianum, which is fairly cold-tolerant. Would it be happier a little warmer? Probably. But it does grow and bloom.

It looks to me that the original plant was too deep in the medium. I have the same orchid, and the "soil" just touched the bottom of the bulb.