Oldest cattleya pseudobulb dry and brown
 

Last Fall, I got this Rhyncholaeliocattleya from an unfortunately inexperienced seller. I can't remember what the specific issue was with this plant but I believe the root system was decimated (guessing by the zip ties I used to hold it in place) and it was in the "ICU" for several months until it started to shoot new roots.

All has been well with the orchid since Spring arrived. Plenty of new roots and a new growth approaching maturity. A few weeks ago I noticed that the oldest pseudobulb got very shrunk and brown.

I didn't think much of it, I just assumed that lacking sufficient roots caused the plant to basically absorb the pseudobulb. However, I wanted to bring it to the forum in case I'm missing something. I know there are some bacterial infections that can look like that and spread through the rhizome, however, I've never had to deal with that so my knowledge is limited.

Looks like it may just be reabsorbing the old pseudobulb. I have a large C. Purpurata that has absorbed some of the oldest pseudobulbs over the few years I've had it, which looked very similar to your pictures at the time.

Looks normal to me. Catts do "cannibalize" old p-bulbs, that dry up and eventually turn light brown. I have lots of those on plants that should have been repotted a few years ago (when the old dry bulbs get trimmed off) . No worries... the kind of rot that is worrisome occurs on green p-bulbs that turn dark brown or black at the top (plant still maybe can be saved) or from the bottom up (where it usually can't) No sign of that here.