Help - rot? on Monn Millenium Magic "Witchcraft"

[chemist]

Hello forum!
I bought this Monn Millenium Magic "Witchcraft" online and received it on June 11. when i received the oldest PB did not show any sign thats something worng with it. But today i noticed that one side is yellow and its kind of soft.
please see pic: ad photo, and todays


Do i need to repot it and separate old PB from others ASAP?
What is your thought what it is?

Thank you !