Incorrect generation of Swagger.json ref using C#

I'm getting incorrect refs from my swagger.json file

For example this method generates an incorrect ref:

/// <summary>
/// Update a Domain
/// </summary>
/// <param name="model">The Domain model to create</param>
/// <returns>the update Domain</returns>
[HttpPut]
public Domain Update([FromBody]DomainModel model)

The incorrect ref generated:

"$ref": "#/components/schemas/MyInterface.Controllers.Api.CustomerConfiguration.DomainController+DomainModel"  

Here is a method that generates a correct ref:

/// <summary>
/// Update a Customers Domain
/// </summary>
/// <param name="model">The CustomerDomain model to update</param>
/// <returns>Return of the updated model</returns>
[HttpPut]
public Domain Save([FromBody] CustomerDomain model)

The correct ref:

 "$ref": "#/components/schemas/MyInterface.Controllers.Api.CustomerConfiguration.CustomerDomain"

Any ideas why this is happening?