Just went through this about a month ago with my 2012 as well. Although, instead of a squeak, my passenger side caliper locked up.
Anyway, someone somewhere thought that using phenolic for the caliper pistons was a good idea.
I think it’s supposed to reduce the chances of the piston becoming seized to the caliper bore(mine still seized).
It’s been my experience that phenolic tends to get more brittle as it ages.
My pistons were cracked, in a very similar manner, and ready to separate when I pulled them out.
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