Ok so I got 2 more double rings yesterday at Frostir, no Krunch.
Today I got another. This time I was careful to kill Frostir using damage, not DOT, and voila, Krunch appeared.
No idea if it's related or anything, this is too rare to perform any real testing. Perhaps devs should look into it though.
Or just leave the double ring not fixed, so we can do more testing.