Hi everyone, new member here!

I am kinda new to house plants in general and I have quite a few right now. I noticed that many of my small but trailing plants seem to have a few mushy stems here and there.

For instance, I have a dischidia oiantha that had this issue. It started losing leaves like crazy, so when I checked, I saw a few mushy stems. I decided to inspect the roots as usually a mushy stem could be due to root rot, but all the roots seemed just fine. I still removed all the cuttings that had the mushy part because I was afraid it could spread to the healthy ones.

But my question is: are all mushy stems a product of fungus activity? A few of my other plants had the same issue, but whenever I inspect the roots, they are all healthy. It's just one part of the stem that gets mushy and very very thin.

If the stem rot is indeed only caused by fungus, should I repot my plants right away? How do I avoid this? I never really overwater them whatsoever. I also try to bottom water as much as I can, but it seems that if I top water just once, then mushy stems will show up within a few days... I didn't want to have to repot my plants over and over even because I can't keep disturbing the roots. Would a peroxide + water solution be able to kill any fungus in the soil so I don't have to repot them?

Thank you very much in advance

Edit: I forgot to mention: they are all in airy soil (perlite, coco coir, orchid bark, activated charcoal and worm casting). I also live in Brazil, and right now, although it's spring, it's been raining quite a bit, so humidity has been high (around 80% if my measuring device is correct). I leave my windows open and when it's not too cold, the ceiling fan as well, so the air can circulate. I've also been having issues with mold on top of my soil for some of the plants... If you guys have any tips, please let me know. I've sprinkled cinnamon but it doesn't seem to help. I'm not sure what else I can do to stop this from happening. I know that fuzzy white mold isn't necessarily bad but it means it's very moist and I'm afraid this could end up facilitating other types of fungus/bacteria