If its the lower rocker panel moulding then no this is not a common issue as far as its come up on the forums.
There was at least once incident where the roof rail molding had blown off someone's car at 70 mph, but that's the only case of plastic parts coming off of the car that had come up here.
It seems more likely that something external would cause the lower rocker cover to come loose/ break than it just falling off on its own, so the onus would be to prove that it was not hit by anything else.
That stinks, but it doesn't seem to be common enough to have even been mentioned here before.
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