LCD flickers and vertical lines

Hi guys,

my iPhone 6 recently displays vertical lines all over the screen, and I can't see anything from the OS, as the background behind the vertical lines is sometimes white and sometimes black. The lcd simply flickers and the lines jump around like crazy :)

Thought it's the LCD but even after changing the screen the same problem persists.

Everything else seems to work as intended as it vibrates on startup and also vibrates when the mute switch is activated.

I'm not sure if it connected with water as my GF was using it when the problem showed up but the LCI doesn't indicate any water damage.

Any ideas what else could cause this problem?

Thanks in advance guys