I have the same issue with my Z3 compact. The issue started happening on the 11th month of my one year warranty, I sent it for repair (14 business days y + shipping time back and forth). It came back with the same issue. Sent it back right away. After another three weeks without my phone, I got it back. Guess what? EXACT SAME ISSUE!
I called their support line explaining what happened, the guy told me that there is nothing they can do about it since my warranty expired. Of course they didn't have in consideration that I sent my phone before it expired and it took about seven weeks back and forth. There wasn't any sort of damage on my phone, was never dropped or damaged in any way, I never submerged it even if in theory, that would be ok. The report said "it has reviewed the functioning of the unit, unfortunately the defect is in the PCB so it is not able to undergo repairs to it."
The guy on the phone told me what he thought the report meant. It means there is damage on the main board(not caused by me) and the one year warranty manufacturer warranty department won't repair it. Maybe, if I had bought the 2 year "extended" warranty, they could do something about it. I asked if the issue is that my one year warranty was expired or if it is something that they just don't fix unless you get the "extended" warranty. He couldn't answer me and the only thing he said was "is there anything else I can help you with?"
From what I understood, there are two completely different departments, the one year manufacturer warranty that only cover some things. Not even sure what, because if it's broken screen, water damage or any physical abuse, they won't fix it. If it's a problem with the main board or PCB, they won't fix it either. It is a mystery to me what gets fixed in that department. The other department is the 2 year extended warranty. I'm not sure what they are willing to fix there, but from what they guy on the phone told me, it's more than the one year regular warranty.
Long story short, after two trips to FedEx, at least 8 hours total on phone calls, more than 7 weeks without my phone, I ended where I started. Sorry Sony, never again. Worst customer support I have ever experienced.