Installed a replacement screen - Vertical flickering issue

See this video: https://streamable.com/ktkw1

This only happens when the screen is turned on, either when the phone is booted up or awaken from sleep. This issue happens for a few mins then it goes away. The screen looks fine afterwards.

Touchscreen functionality is OK.

I would like to know if this is a defect in the replacement screen or something I messed up during installation i.e. connection etc?

Can someone also explain what is causing this issue? And why does it go away after a few minutes?

Thanks!