What's happening with Monnierara Millennium Magic Witchcraft?
 

Hello!
Can you tell me how to rescue Monnierara Millennium Magic 'Witchcraft' (if it needs a rescue)?
I purchased this plant about a month ago. The bulbs were green, but there were no leaves or any new buds (currently absent as well). It was planted in moss, which practically decomposed, so I decided to transplant it. I think I damaged a lot of roots during transplanting because it was a very dense ball of moss and roots. And unfortunately, the plant split into two parts (in one there are 2 bulbs, in the other 3). During the transplant, I did not notice rot, although I could simply not understand whether there was, because everything was beige and brown.
As a result, about 2-3 weeks after transplanting, the top of the largest bulb began to turn brown. I cannot understand whether this is rot or natural dying, but it creeps down rather quickly. Should I cut off half of the bulb, or cut it whole, or leave it as it is? What is the best way to process a cut? Any other tips would be very helpful.
First two pictures before repotting. Others after.

An orchid is like an iceberg.
Not literally but just like the Titanic did not run aground on the Ice berg it could see, it scraped past the 2/3 of the ice berg submerged under the water they could not see.
An orchid has a lot of roots under the surface and if there is a problem in the root zone 99% of times it will show on the surface above.
Gorwing orchids however is not an exact science and that is my assumption and advice. Check the roots before you make any further decisions on what to do.

Edit: reading over the post again you do mention the roots were all mushy and brown and sound like they were on their way out and a leafless, rootless orchid is not my expertise so I will let someone more experienced handling recoveries offer guidance on how to save it.

Is the spot mushy or firm?

this orchid is in dormancy.

i would clean all the loose parts of and then re examine the brown spot- I agree with clawhammer that the firmness is a HUGE indicator- hard, leave it, mushy, cut below it with a single use razor and then dust with cinnamon.

while it is not how i grow these plants, i would remove all the debris and then lay the plant in a warm and humid space on a bed of sphag until it wakes up

Did you water it? Watering these during dormancy, especially after cutting roots, can kill them. It was not in great shape when it arrived.

Go to Sunset Valley Orchids and read about how to grow these.

For now I would pull it out of that container, shake off all the media and dry it out as fast as possible. Put it someplace warm and humid, and hope it begins a new growth. The information at SVO will tell you when to water.

Thank you for suggestions!
Indeed I watered it once when reporting, cause I wanted to get rid of the moss under running water.
Yes, I will remove it from the container and dry it, then put it somewhere on the moss in a humid environment. It seems strange that it is dormant in the middle of July though.
Will wait with cutting the cane and see how it goes (who knows, may be it is sucking nutritions from it now, I can cut it later). It starts becoming a bit mushy on top though.

Did you import the plant from another country? If it's a local vendor, is there a chance that they recently imported the plant? A dormant plant in the middle of summer would usually be seen if the plant was originally growing in the other hemisphere, where is would be dormant now.

IF it is mushy it will not recover and the mushy area will likely expand. If you see any expansion of the mushy area cut it off as described by DC. Plant won't miss a beat.

It looks like rot to me. I would cut a bit below the yellow with a clean razor (such that there is no brown on the inside left). Catasetum types have degraded roots during dormancy, but not dead. New roots could grow out of the old ones in the next growing season. The plant shouldn't be watered until it gets a few inches of new growth and lots of roots. You may place it in a dry area with low light until new growth initiates, then move it into higher lighting.

Thank you for your advice. I doubt that this orchid had been imported recently. I bought it at some kind of online flea market here in Japan. But indeed I find it strange to be dormant in the middle of July.
So I checked the roots again, they were all hollow, and I decided to cut them after they dried. I noticed a couple of tiny (hopefully) sleeping buds (one on each plant), so I will leave it on dry sphagnum moss on a shady window for now and will wait for the miracle to happen.
The brown top on one of the bulbs became much thinner and almost completely dried, so maybe it's better to wait with the cut for now.