What's the reason of this display trouble?

This looks like a faulty graphics chip on the logic board. You have 3 options to repair this:

• 1- Replace the entire logic board.

• 2- Perform a reflow on the faulty graphics chip with a heat gun. Check out this old post: video graphics and boot issues

• 3- What i have done that works best is to perform a BGA reballing on the graphics chip but you need specialized tools and skills with soldering to do this and can be more expensive than replacing the logic board

What I would do in your situation is attempt the reflow first to see if it works. If it doesn't then I would just replace the logic board. See this guide: iMac Intel 17" Logic Board Replacement

If you want more info on reflowing or reballing try a google search of GPU reflow diy, BGA rework.

I hope this helps, good luck!