Hmm.. that's an odd one.
Rarely having used Michael's Castin-craft and never having used Testors in my pens, I'm guessing here.
I'd say the issue is that the castin-craft is getting hotter as it cures, softening the testors. Either that or it has a chemical in it that is softening the Testors. The fact that your Rust-Oleum was high temperature and so should be tolerant of those temps is why I say this.
You didn't by chance open a new bottle of MEKP on this batch, did you?