Cactus starting to lean to one side- root rot?

[Flapjack Octopus]

For a year or so, I have been raising cacti from a mystery seed mixture that I bought at the local succulent collection. I am fairly new to cacti and lack the knowledge to identify the tiny cacti I am growing or any issues that they might have. In short- I am an idiot when it comes to cacti. Recently I have noticed that one of my seedlings has started to lean to one side. I unearthed and cleaned it, and as you can see it looks as if someone took a bite out of it. The caved-in part is reddish brown in color and not mushy, but dry and shriveled (Might be irrelevant, but there were also tiny pebbles wedged in there).

Could that be root rot even though it is dry? Is my cactus doomed?

[Flapjack Octopus]

I am having weird issues with the attachments (Uploaded the wrong files, deleted them, attatched the right ones but the old ones (that i deleted) reappeared upon submission even though the preview did not show them?) Second try

[ElieEstephane]

There may have been rot but it looks like it has stopped of it's dry. Use your better judgement to see if it's completely dry and healed.
You probably have done more damage unearthing it. It could be some sort of coryphantha

[Flapjack Octopus]

There may have been rot but it looks like it has stopped of it's dry. Use your better judgement to see if it's completely dry and healed. You probably have done more damage unearthing it.

I was afraid so, but i did not see any other way to determine why it started to slant like this and if there was an ongoing infection or not It seems dry. Should I treat it with a fungicide or something to prevent reinfection?

It could be some sort of coryphantha

I will keep that in mind and see how it looks like when it (hopefully) gets older. Thanks for the help!

[ElieEstephane]

Should I treat it with a fungicide or something to prevent reinfection?

Yeah why not. Even if it's not infected, it wouldn't hurt. When you replant it, hold off watering for a few days to avoid root rot.