Doxygen warning "does not have an associated number"

Question

I am using doxygen 1.8.9.1 and bibtex to generate the reference list. It looks doxygen (or bibtex) don't support longer key.

For example, my bib file looks like

@incollection{mcmaster_phenology_,
    title = {The {DSSAT} cropping system model},
    volume = {18},
    journaltitle = {European Journal of Agronomy},
    author = {A, F.},
}

@article{mcmaster_phenology_1,
    title = {The {DSSAT} cropping system model},
    volume = {18},
    journaltitle = {European Journal of Agronomy},
    author = {A, F.},
}

My text.cs file looks

namespace Test
{
    /// <summary>
    /// A test class
    /// </summary>
    /// <remarks>
    /// This is test citation, see \cite mcmaster_phenology_ and 
    /// \cite mcmaster_phenology_1.
    /// </remarks>
    public class TestClass
    {
    }

}

The warning message is:

warning: \cite command to 'mcmaster_phenology_1' does not have an associated number

The final html is

This is test citation, see [1] and [mcmaster_phenology_1].

Seems the maximum length of key is 19 characters. It looks a bug for me. How should I fix it? Thank for any suggestions.