Ponytail Palm 'Bulb' Rotting?

[Oubadah]

I have this Ponytail which I moved from a pot into the ground about 6 months ago. Since the move, I've noticed something strange happening in the 'bulb' (caudex?). The outer layer (~7mm deep) seems to have rotted out, however the bulb beneath the rotted layer still feels very firm and the foliage looks healthy as pictured below.

It's in a very well drained area, so I can't imagine why there would be any rot, and I've planted several other Ponytails from pots into the ground in the past, with no issues whatsoever.

I just want to make sure there is no further rotting. Has anyone seen this before:

[img]https://I.imgur.com/Rud7A.jpg[/img]
[img]https://I.imgur.com/ewkhH.jpg[/img]
[img]https://I.imgur.com/TdjAR.jpg[/img]

Any input would be most appreciated.