Since sadly an upgrade to 2021 is currently not possible to us and there seems to be no intention to include the fix into 2019.4 LTS (!sic), here is a Input Action processor that can be used to work around the issue:

#if UNITY_EDITOR
[InitializeOnLoad]
#endif
public class WebGLScrollProcessor : InputProcessor<Vector2>
{
#if UNITY_EDITOR
static WebGLScrollProcessor()
{
Initialize();
}
#endif

[RuntimeInitializeOnLoadMethod]
static void Initialize()
{
InputSystem.RegisterProcessor<WebGLScrollProcessor>();
}

[Tooltip("ScrollValue replacement for incoming Scroll Vector.")]
public float scrollValue = 120f;

public override Vector2 Process(Vector2 value, InputControl control)
{
if (value.y > 0)
{
value.y = scrollValue;
}
if (value.y < 0)
{
value.y = -scrollValue;
}
return value;
}
}