I think the case when ProjectWorldLocationToScreen returns false needs to be treated explicitly. In my tests the screen coordinates were always 1 when the method’s return value was false.
Sam sais that it’s ok when the world location is off screen because we can have the tunnel centre off screen. But 1/1 is not off screen, it is just in one corner. As with many boolean functions that give you further results in ref or out params (think of typical raycast functions), I think you are not supposed to take those params as meaningfully set when the return value was false. Maybe the poster of this issue encountered the same problem, as lateral movement (in the sense of strafing) is the prime example where you get an off-screen world location.
Could also be that the method implementation was changed since Sam’s recording; I am using UE 5.3 currently. Unfortunately, the docs of the method are not really verbose. A quick glance into current engine source code seems to support my results.
(Concerns lecture “Creating Tunnel Vision” from the Unreal VR course).