Hey everyone i hope everyones plants are doing well!

i have this astrophytum that seems to be corking really fast. The brown line keeps moving up. last time i repotted it the corking was maybe a centimeter or two max above the gravel. Now its easily more than an inch in certain places.

I was wondering if this is normal? Cuz it sure aint pretty :(

the brown part is still firm and i cant easily pull the plant from the pot so i dont think its rotting from beneath(Not that i water it a lot either) Its in full sun in a s-w window. Porous soil and terra cotta pot etc

thanks a bunch in advance for any advice