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I'm sorry to hear that but if there is/was physical damage to the phone and/or if the covers weren't firmly closed then water could permeate, how to know for sure, contact your Local support and request a vacuum test, it will prove who is at fault.
if the back cover started to open then you should've stopped by your Local support then, perhaps you never noticed since you had a protective cover, I'm sorry to say this but there's nothing we can suggest here.
@JeromeC1 wrote:
It was not physically damaged since I always used a protective cover but after water got in now the back of the phone started to "open" ...