I think those strokes are caused by back lash on the z-axis

The coupler I am using is rigid 3d printed

The Z-coupler looks good visualy and torquing by hand , but I think it start slipping causing small back lash.

Then It slipped badly causing the last part of the workpiece to be completly removed. 

I ordered few couplers. Waiting for delivery.

The second picture is using another machine

Philip

 

 

Plastic couplers will slip, short of drilling a hole through the coupler and shaft for a pin/screw, they will not be usable. Also stepper motors get hot so the shaft gets hot, causing the coupler to soften.
PLA softens at 50 to 60 degree C, PETG at 70 to 80 C, ABS at 90 to 110 C, HIPS at 105.
I used HIPS for some 3D printer parts, it is a nice material, very strong and stiff, but warps like crazy if you just look at it funny.