I have two pairs. One is 5318 and one is 5216. The 5318 does not have this problem but the arm of the 5216 is erasing.. Is this a common problem? The 5318 is older but it seems to be printed with different material. No questions of authenticity since they were purchased at solstice and nordstroms.
That's pretty normal , specially if the lettering had a closer contact to your skin .
Our body oils , plus the heat and water are pretty much very harsh , overtime this lettering will fade away . Just enjoy the sunglasses , no one will see them anyway .