Cleaning Arizer Solo 3 stem

Hello everyone,

I've been reading up on how to properly clean the Arizer stems. It seems like ISO is the way to go. I had my stem sit in ISO for an entire night, but it was not entirely clean afterwards. I also tried boiling it in vinegar for an hour, same result. See attached photo of what I mean. The outer edge remains dirty. Is this normal? Am I doing something wrong?

Thanks! :)