Bullet Spread Yaw changing when aiming upwards

I’m trying to make consistent (and circular) bullet spread, however when i aim upwards the bullets no longer deviate in the yaw axis anymore.

The system first randomly calculates an amount to miss by, and an angle to miss in(polar co-ordinate based so the spread is circular), it then converts that into an FRotator (in the Pitch and Yaw axis) and adds this to the Spawn Rotation to determine the end point of the debug line draw, Ive also included a debug sphere draw to clearly demonstrate how the spread amount changes as the aim pitch changes
So here’s my code:

float Spread; //Picks a random amount to miss by
Spread = Fire1SpreadMin + FMath::FRand() * Fire1SpreadMax; //SpreadMin is for donut shaped pattern
float SpreadAngle; //Picks an Angle Perpendicular to the firing direction to spread
SpreadAngle = FMath::FRand() * 360.0f;
//Combines these into a Rotator Vector to add to the Spawn Rotation.
FRotator RandAngle;
RandAngle.Yaw = YawSM * Spread * FMath::Sin(SpreadAngle);
RandAngle.Pitch = PitchSM * Spread * FMath::Cos(SpreadAngle);
//Debug line Projectile Trajectory
uint8 col;
col = rand() * 255;
DrawDebugLine(	World, 
			SpawnLocation + (100000.0f * (SpawnRotation + RandAngle).Vector()), 
			FColor(255, 255, 0), //Yellow
			false,	//Persistent?
			0.0f,	//Lifetime
			0,		//Depth Priority
			0.0f);	//Line Width

			SpawnLocation + (5000.0f * (SpawnRotation + RandAngle).Vector()),
			4,	//Segments
			FColor(255, 0, 0),
			false,	//Persistent
			10.0f,	//Duration
			0,		//Depth Priority
			0.0f);	//Width

Here are some pictures to illustrate my problem
Aiming Horizontally

Aiming Diagonally

Aiming Vertically

So how can i get the spread to be independent of the angle the player is aiming?

Did You found a solution ? Please share I have the same problem.