Entity Framework lazy loaded collection sometimes null

I have 2 models, one of which has a child collection of the other:

[Table("ParentTable")]
public class Parent
{
    [Key, Column("Parent")]
    public string Id { get; set; }

    [Column("ParentName")]
    public string Name { get; set; }

    public virtual ICollection<Widget> Widgets { get; set; }
}

[Table("WidgetTable")]
public class Widget
{
    public string Year { get; set; }

    [Column("Parent")]
    public string ParentId { get; set; }

    public string Comments { get; set; }

    [Key, Column("ID_Widget")]
    public int Id { get; set; }

    [ForeignKey("ParentId"), JsonIgnore]
    public virtual Parent Parent { get; set; }
}

This code works for > 99% of widgets:

var parent = _dbContext.Parents.FirstOrDefault(p => p.Id == parentId);

Usually, parent.Widgets is a collection with more than one item. In a couple of instances, however, parent.Widgets is null (not a collection with no items).

I have used Query Analyzer to trace both the query for the parent and the query for widgets belonging to that parent. Both return exactly the rows I expect; however, the model for one or two parent IDs results in a null value for the Widgets collection. What could cause a lazy-loaded collection to be null in some instances but not others?