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The phone can't overheat, if it gets too hot it will shutdown, with that said, it "could" overheat if you run a Custom ROM or if you have mod or overclocked the phone.
Contact your Local support to get it inspected.
I haven't experience any of this with any of my Xperias, just let your Local support inspect your device.
@Prso9: it takes a heat gun just to loosen the adhesive used to bind the back panels. https://www.youtube.com/watch?v=cBMxYINFyjo
so if you experience that kind of heat it should have melted the case you are using,. I have a Z5C mostly the heat is concentrated around the middle of the back and not on the edges..
I agree with the suggestion above. please see the nearest service center to have this fixed/
"The phone can't overheat, if it gets too hot it will shutdown" have you considered that it might not shutdown as it is not working correctly? The backplanel on my Sony Xperia z5 has come off around the camera area. It gets so hot that I cannot have it in a pocket as it feels like my leg is burning through the fabric. It has not been tampered with and is always in a case so is carefully looked after.
I have the same problem where the top section of the back glass is separating from the body of the phone. I know it's because of the excessive heat emanating from the top back area that has melted the glue.