I actually have the same problem on my Z2 , i previously owned the Xperia Z where i was experiencing same issue, and yes is covered under warranty....glass becomes unglued and separating from frame because NFC pins pushing the glass out, and phone generating heat during usage and this way the adhesive sticker inside becomes melt and glass starts to go out...this is a known issue even Sony knows about. I was able to hold 5 different Z2s and every one had the same issue, same area, different batch numbers.
The fifth one was yesterday in Orange ( operator) i dont know how to cerrectly call this showpiece? I mean the place where you can grap the phone and play with it... I noticed this problem on this phone....
I was analyzing this problem and i found that first area where it starts to come off is where the big flap is, thats the place where NFC pins are, and it continue to ungluing to the corner exactly like it is on your photo....Come on Sony!! Z1 was proof from this becasue of the middle frame and, this way nothing is pushing the glass out and also adhesive sticker is more wider.... I know what i am talking about i am gsm mechanic i was assembling both devices. But i dont know if you guys understand my English
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