When an element contains an attribute that has default values in its constructor it will no longer appear in the UI Builder or be serializable from UXML. 

The issue can be seen in the Editor log file where an error occurs in the source generator. 

1) Add the following script to a project:
{code:java}
using UnityEngine.UIElements;
[System.AttributeUsage(System.AttributeTargets.All)]
public class DisplayNameAttribute : System.Attribute
{
    public string Name { get; set; }
    public string IconPath { get; set; }
    public DisplayNameAttribute(string name, string iconPath = null)
    {
        Name = name;
        IconPath = iconPath;
    }
}
[UxmlElement]
public partial class MyElement : VisualElement
{
    [UxmlAttribute, DisplayName("my name")]
    public float myField;
}
{code}
2) Notice the element does not appear in the UI Builder Library. The Editor log file will contain an error message, however it wont be visible in the console.

3) Remove DisplayName from myField.

4) Element is now visible.