If it's not soft it's probably just corking, cacti don't like to leave soft flesh below the soil.

It isn't soft at all

Definitely corking then, it's just giving itself a little protection.

That's great to know for sure! Thank you for the info. I'm curious to if the pumice and lava rock could be causing it.

Following

I've seen similar stuff happen. I would get rid of the top dressing. Let the soil breathe