Insert returns 0 when using a string as an ExplicitKey on an object

Topic, this may also be related to DapperLib/Dapper#756 but i'm not sure if it's the same issue.
Here is the object I am using:

using Dapper.Contrib.Extensions;

[Table("CostBooks.Staff")]
public class Staff
{
    [ExplicitKey]
    public string EmplNbr { get; set; }
    public string AlphaName { get; set; }
    public bool? IsFte { get; set; }
}

Here is the method I am calling. Will try to do this with async and see what happens.

[HttpPost]
public object CreateNewStaff([FromBody]Staff s)
{
    using (IDbConnection db = DBAccessor.Connection())
    {
        if (s.EmplNbr != null)
        {
            var emplNbr = db.Insert<Staff>(s);
            return emplNbr
        }
        else
        {
            return null;
        }
    }
}

Start by reproducing db.Insert(s) with the Staff model shown, especially its string [ExplicitKey] property. Inspect the insert behavior for explicit keys and determine why the call returns 0; done means the insert returns the expected result for this model and a regression test covers it.