This was closed a long time ago, but we're still experiencing this behavior.

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;
}
}