Division problem: What am I doing wrong and how can I fix it?

[BrassavolaStars]

Hello all,

I have an urgent orchid issue. I divided this rare Cattleya maxima and now it has a black spot where the cut was made.

I made the division to remove a rotten bulb and cut it back until there was green, and then left the plant out of the pot for several days to let the cut callous over. Then, I put it in a new pot with new medium. Yet, now there is more rot.

I think I definitely need to take it out of the pot again and cut this section off.

What can I do this time to make sure it does not happen again?

I do not want to slowly lose this plant from bad division after bad division. It has a lot of dried-out, papery roots.

Should I remove those this time? Maybe root rot is the issue. I also have Alliette and Heritage.

I used the Heritage the first time.

Thank you.