I hope a cactus expert chimes in, but I'll give you my observation. I think you may have rot, or at least compromised roots. It doesn't always effect every plant the same. For example, I had 4 Crassula capitella 'Campfire' plants in a pot. One rotted, the others were fine.

Is that a plastic liner in your pot? That makes me question the drainage for the pot. You must have good drainage.

The soil looks heavy to me. Most of us amend the soil with pumice, grit or perlite by adding at least 50-60% to the soil. This improves drainage. I use Al's Gritty Mix. You can search this forum for that and other "recipes" for potting mix.

If I were you, I would unpot the plant and look at the roots.

that pad is dead/necrotic at the base… you should cut the pad off above the rot and let it dry out… should reroot again.

Im not a cactus expert but it looks like its rotting its way up. If that were my plant, I'd take it out of that medium, get a sterile or disposable blade, cut about an inch higher than the dead tissue. Make sure when you cut through there is no additional necrotic tissue. Whenever you make a new cut, gently wipe the blade with alcohol to avoid spreading anything. Sulfur powder is great to dab on the open wound if you have it, if not its ok. Let it dry in a place with indirect sunlight till it callouses over, then pot it up.