Iphone 7 in constant reboot mode

Hi all, i had this issue with my iphone 7 where it's stuck in reboot mode. The apple icon comes and goes non stop.

All i did was update it to the latest iOS 11.2.6 on Sat night and Sun night it went into the reboot mode. I took it to a shop to check today and they tried to restore/reinstall OS but it didn't work. There's also some intermittent lines on the screen once in a while.

Now they suspect its the motherboard and they will send it for further check. I'm really surprise because the phone is only 1 year and 3 months old.

Anyone had similar issue and know the cause?