I have seen this a number of times. When one fails, most of the time, more will follow. If so, just take out the cam and rockers and heli-coil them all. I have done this and when done, the threads are stronger than stock.
The problem is the iron or steel bolts and aluminum threads. Mercedes had the same problems with the blocks then went with all chromoly bolts to cure the problems.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS