It's almost assuredly a result of the hole being too small. If it were the plastic's composition that's the problem, his nosecone, tail fins elbow joints would likely crack as well, but they haven't.
Since it's a Starscream-only problem they probably tweaked the molds before producing any other seekers.>>6664417
I don't think that would cause it, I have cracked knees on mine despite being correctly assembled.>>6664423
The armature holding the back panel has been installed backwards on early versions. If the clip faces downwards instead of up it's misassembled, and that also keeps the back panel from going flush in jet mode.