Are you sure you did the repair correctly and used the correct type helicoil?
I've used helicoils on MUCH higher pressure/heat situations than that and had no problems.
My father in law's buick is one that I can definately attest too. 3.4 v6's had an issue with stripping rocker bolt threads when removed or when improperly torqued.
I helicoiled 2 while doing a lower intake on it about 4+ years ago at 40k miles.
It was passed to his sister and now has over 160k on it with no issues.
Could you have possibly used soft incorrect type helicoils? Al has a low melting point ~1400*,but still above combustion temps (obviously as the head is aluminum).
I've helicoiled several plugs in auto engines and never had an issue.
You used the correct type threadlocker right?