What does this Azure Search indexing error mean? "The property 'x' does not exist on type 'search.documentFields'..."

The Exception I am getting is: The property 'documentType' does not exist on type 'search.documentFields'. Make sure to only use property names that are defined by the type.

I have googled this and still cannot figure out what is going on.

Here is the model we are using:

[SerializePropertyNamesAsCamelCase]
public class WebSearchDocument : SearchDocument, IEventSearchDocument, IResourceSearchDocument
{
    [IsFacetable]
    public string DocumentType { get; set; }
    [IsSearchable]
    public string Title { get; set; }
    [IsSearchable]
    public string Description { get; set; }
    [IsFilterable]
    public DateTime? PublishedDate { get; set; }
    public DateTime? LastUpdatedDate { get; set; }
    public string ImageUrl { get; set; }
    public string LinkToResource { get; set; }
    public string EventCode { get; set; }
    [IsSearchable, IsFilterable]
    public string Location { get; set; }
    [IsFilterable]
    public DateTime? EventStartDate { get; set; }
    [IsFilterable]
    public DateTime? EventEndDate { get; set; }
}

And finally, this is the index fields on Azure

Per @Bruce Johnson request, here is more information

Service Name: bacp-search
Index in question bacp-web-dev
We are using Microsoft.Azure.Search 3.0.3 (NuGet)

SearchDocument Base:

[SerializePropertyNamesAsCamelCase]
public abstract class SearchDocument : ISearchDocument
{
    /// <summary>
    /// Gets or sets the ID for the document. For consistency, this should never be updated or retrieved manually.
    /// All IDs should be set through the <see cref="Id"/> property.
    /// </summary>
    [Key]
    [JsonProperty("Id")]
    public string AzureId { get; set; }

    /// <summary>
    /// Gets or sets any unique IDs or compound IDs that might contain characters unsafe for transmission via URL.
    /// For consistency, all IDs should be set through this property.
    /// </summary>
    [JsonIgnore]
    [IsRetrievable(false)]
    public string Id
    {
        get
        {
            return AzureId.FromBase64EncodedString();
        }
        set
        {
            AzureId = value.ToBase64EncodedString();
        }
    }
}