Please help diagnose my Rebutia…

Hi lovely people! First time cactus owner here looking for some advice. My small Rebutia cactus has been developing these crusty lesions and blisters for a few weeks, which is now showing on some of the pups too. What is happening? It was flowering last month so I thought it was doing well… The Rebutia is in the same pot with two Mammilaria cacti (they seem healthy to me) - should I remove the Rebutia from the pot?

For context I live in Australia and the plants are kept indoors. I bought them two months ago in separate pots and have recently replanted them together. Roots looked ok to me when I replanted and I have only watered them once. I live in an apartment with quite poor ventilation and minimal indirect sunlight, and I keep the thermostat between 19C to 22C (66F to 72F). If it is relevant, the plants can be moved to a shaded outdoor balcony, but sunlight will still be minimal unfortunately. Thanks in advance!

 

Poor ventilation and poor light quality suggests a fungal infection to me. I hope I won't offend you but your home unfortunately does not sound an ideal place for cacti.
Also it could be something nibbling at the flesh of the cactus, like a small insect maybe.
If you could place the cacti outside I think they'd grow better even with a bit of shade, but good ventilation is important for cacti.
Good luck ! I hope the problem can be cured, and hopefully someone will come up with better suggestions than I have

 

Equally cant be sure, maybe go over the plant with a hand lens and look for bugs attacking the plant, it could be fungal but somehow growing indoors I wouldn't have thought so.
Rebutias are usually one of the easiest cacti to grow, so not sure.

I dont think its lack of light as the plants usually look a bit stretched if that is a problem.

It doesn't look terminal anyway, to me, and it might just grow out of it.