Custom created Sales Campaign does not generate TinyMCE correct

Hi!

When extending the default SalesCampaign with an extra TinyMCE the property is not rendered correctly in the edit mode.

[ContentType(DisplayName = "Extended campaign", GUID = "7a3b14a9-8b3b-4064-8bef-7d6ddb11a556")]
[AvailableContentTypes(Include = new[] { typeof(PromotionData) })]
public class ExtendedSalesCampaign : SalesCampaign
{
    [Display(Order = 15, Name = "Description")]
    public virtual XhtmlString MainBody { get; set; }
}

I get the same behavior in Foundation and it looks like my extended properties and rendered with the css class: epi-grid__col--6

If I change to epi-grid__col--12 it looks just fine. Is this something I can change?

Thanks!

/Kristoffer