I believe RPD (rapid plastic degradation) is triggered by protective layer of human skin.
Its most likely due to a genetic difference that a big group of people seem to have.
Some have this issue on most of their devices, while others can never encounter it unless they buy used.
People with skin oil that triggers RPD most often report on soft touch and rubbery plastic surfaces being affected the most, due to less dense structure and thus more surface area on the molecular level, making those materials way more vulnerable to the start of the reaction.
Sunlight, all be it - not a catalyst, acts as amplifier once reaction had began.
For OP i highly suggest getting new buttons asap from Elektron support.
There is a chance you can contaminate your other devices, even if you manage to clean them.
And yes, it would be cool to finally figure out whats causing it for sure, and engineer new materials that can be used by everyone.