Yes, welcome, and I think Donald has offered some valuable advice. On top of that, I would add a few observations.
Rot is generally a one way street with my plants, especially when it affects the stem. I don't know if there is a reliable treatment (after some point I know there is not) so much as useful measures of protection.
Sometimes it is possible to identify the cause of a plant going mushy, other times it's kind of guesswork. But generally the issue relates to insufficient light, excess water, insufficient air flow. To the extent you can address any of these possible root causes, it's a good idea to revisit the care you are giving in order to fine tune it as appropriate.
Also, as a general rule, it's not a good idea to bare root your small/baby cacti any more than necessary. They are vulnerable at that size and may respond poorly to damage or loss of roots. So extract useful information if you need to, otherwise leave the roots well enough alone. You may end up causing harm to the plant.
Any time you handle the roots (bare rooting, repotting, invasive handling) it's a good idea to avoid water immediately afterwards. Wait a week or whatever first. There is a real risk of rot if damaged parts don't have a chance to heal before they encounter dirty water.
