Pseudolithos disease, a dark spot o n the plant body

Three months ago a dark spot appeared on a body of my Pseudolithos plant.

I thought it was fungal infection, immediately carrantined the plant to prevent infection spread on my other plants.

I treated the sick plant with biofungicides such as Bacillus subtilis and Trichoderma veride.

So, after three months dark spot didn't increase in size, also it dried out and become like a cork.

The plant is hard, not squishy and keep growing.

What should I do next? Was it a fungal disease? The plant still in a quarantine, grows on a window sill, there is no sufficient sunlight for the plant now, because of autumn.