Strange behavior in fullscreen mode

I have a monitor 24" 1920x1080 (Full HD)
My development computer is an Acer Aspire Nitro V15 i7 12 GB with a Geforce 950m.

If I switch to fullscreen in the game settings then I have extreme fluctuations between 30-60 frames when I move the camera (in a first person controller).
No matter if I use 1920x1080 or 1280x 720 or 1024x768

In window mode it runs constantly at 60 frames.
It doesn’t matter whether I use 1024x768 or 1280x720.
1600x 900 also runs in the window Always with 60 frames.

Does anyone have a tip for me?

In the Unity Engine or Unreal it makes no difference whether it runs in a window or fullscreen, the frame rate is always constant.