Is the stalk soft or mushy?

No, it's hard.

I'll be interested in hearing what it is. I have a blue sky that did something similar. It's in gritty mix and definitely not rot. It's hard too. The feedback I received was that it was likely burned from excessive heat in a too small pot. But yours is indoors so it's can't be that.

If it was over watered couldn't that cause leaf loss? But wouldn't that affect the stem as well? Can they burn even on a window sill? I'm still learning so I ask for my own general knowledge, not questioning your care practices Jess;). I'm keen to find out the cause,,did the leaves turn pink orior to falling off or before? sorry I can offer no proper help but I'm intrigued by this

I think it is a fungal infection. Notice that the only leaves remaining are those contacting still green stem. It does not have to be soft to be rot, at least not early on. Remove that plant from the pot immediately and have a closer look. I don't think that one is going to be salvageable, but you may be able to propogate new plants from the few leaves that don't have discoloration at the base.

Brad