I have the same situation here. I also bought my phone 6 months ago and I tried taking underwater pictures for the first time a week ago, in depth even less than 50 cm. So after that I have had a lot of condensation under the screen and the cameras, then my phone just shut down and wasn't responding at all for 24 hours. When I tried to turn it on BV started bootlooping and then no response again (for 72 hours this time). Today my BV booted into system successfully but shut down 3 minutes later with an error message "your battery temperature is too low. shutting down"
So is there any solution or shoud I also ask my seller to replace it ? I doubt it would be possible to be honest
Unfortunately, you'll have to contact your seller.