The gradient color fill is weird of Line.
In a specific perspective:
:(
Reporting a bug? please specify Unity version:
2021.1.15f1c1
Reporting a bug? please specify Shapes version:
4.1.0
Reporting a bug? please specify Render Pipeline:
Built-in render pipeline
this is unfortunately a limitation in how rendering works with transparency and draw order, but you can solve it by using the opaque blend mode instead of transparent