It does not look like natural corking to me.
When a Euphorbia forms cork 'bark' it is usually from the bottom up.
Your brown is skipping around with green between the brown spots.
The brown areas look shrunken. Corking does not do that.
I believe it is a fungus. I see concentric lines behind the leading edge of the brown .
These could be signs of anthracnose.
You should cut off the damaged parts and make new starts from the clean growing tips.
Be sure to throw away the dirt they were grown in. Do not put into your compost.
Throw away the pots or soak them in Bleach.
Sterilize your cutting tool after use.
Copper or sulfur spray is possible to use as a protectant, not a cure.
