Give your trunk and branches a gentle squeeze. If they give, are soft and mushy, you have an internal rot. If they are firm, you might be okay.
To me this looks like it was kept too wet while at the same time being kept too cool. Wet & cool will trigger rots of various types.
As soon as danger of frost is past I would get this outside in full sun. The UV in the sun has a anti-microbial effect. Good air movement helps prevent issues too.
Once a rot starts, it is very difficult to stop. Best is prevention. When I had one, it would get zero water from October through February, while the house was cool. I would resume watering in March. Leaves would drop sometime after I quit watering, then a new set of leaves would start after I resumed watering.