Was the indent centered, or off center?
If it was centered, I'd agree that there was possibly either debris in the firing pin channel or a weak firing pin spring, but this seems unlikely if it always occurs only on the first round of a magazine.
If the strike is off-center, that indicates the slide was slightly out of battery, or slightly "unlocked." This could be caused by improper chambering of the first round ("riding the slide" forward) or a slightly weakened recoil spring that is slowed by the tension that is placed on the top round in a fully loaded magazine, or by anything that impedes the movement of the slide on the first round firing sequence.