These photos are much better detail and show the problem.
I do not believe that this is corking.
That spot on the back side is suspicious.
Surgery may be the only solution.
I hope others see these photos and can give you a better analysis.
Chopping off the head is easy to do, but timing the rest period
and re-rooting the cutting is not so easy.
This cutting will need time to form a callus. Maybe a month or even more.
Then you start the rooting process by standing it on top of the dirt mix.
I use several wooden/bamboo stakes to support it, to keep it from wobbling during the rooting process.
It might be wise to plan the start of rooting to be in the next active growth period, like next spring.
I am not an indoor grower, so do not feel comfortable with advising when to do it.
If more spots appear in the upper growth then you should not hesitate.
