Handset has known problem-still have to wait!

I recently purchased a Sony Xperia M5. The device switches off by itself even though there is lots of battery, and the only way it can be switched on again is by plugging it into the charger. Upon further research this is a known problem of this device, but yet I have to hand in the phone for it to be looked at. So I am inconvenienced for a device that is known to be faulty. When I try to argue this point the consultant I spoke to at Woodlands was like a stuck record - no initiative whatsoever to help me! It's one thing to have to hand in my phone because of a fault of my own, but when the device is being sold with a known fault I should NOT be the one inconvenienced.