It may have already been asked, but do you hold the Switch with one hand? Maybe it's putting too much stress on it if so. I stopped holding mine with one hand because the length of the device made it feel like there was extra pressure on the device when holding it that way.
Before I get attacked, I'm not saying that it should, just trying to understand what may be causing to so others can avoid.
This isn't to be ignored, stress fractures. I was guilty of the same thing initially and I had a crack too.
Quoting myself from page 1 of the thread:
OP by any chance left handed? This looks pretty much like a stress fracture by the pressure of holding the Switch by it's Left joy-con one handed (and thus bending the unit ever so slightly via the rail that connects the two pieces).