Database column defaults not behaving as expected

Question

I have an issue where writing a new entry to the database isn't using the configured default values as I'd expect.

I have some classes:

public class Benefits
{
    public string Id { get; set; }

    [Required, ForeignKey("Id"), JsonIgnore]
    public Employee Employee { get; set; }

    [Column(TypeName = "money")]
    public decimal BenefitsCost { get; set; }

    [Column(TypeName = "decimal(16,8)"), DefaultValue(1)]
    public decimal Modifier { get; set; }

}

public class BenefitsEmployee : Benefits
{}

public class BenefitsDependent : Benefits
{
    [Required]
    public string NameFirst { get; set; }

    [Required]
    public string NameLast { get; set; }
}

In OnModelCreating:

builder.Entity<BenefitsEmployee>().Property(b => b.BenefitsCost).HasDefaultValue(1000);
builder.Entity<BenefitsDependent>().Property(b => b.BenefitsCost).HasDefaultValue(500);

And, as last step in Configure, roughly:

var user = new {
    Id = "SomeId",
    NameFirst = "FirstName",
    NameLast = "LastName",
    SalaryBase = 0,
    BenefitsCost = 0,
};

var emp = new Employee
{
    Id = user.Id,
    NameFirst = user.NameFirst,
    NameLast = user.NameLast,
    SalaryBase = user.SalaryBase,
    BenefitsEmployee = new BenefitsEmployee
    {
        BenefitsCost = user.BenefitsCost,
    },
};

await context.AddAsync(emp);

await context.SaveChangesAsync();

The problem is that BenefitsCost is set to 1000 and Modifier is set to 0 rather than 0 and 1 as I'd expect.

What might I be doing wrong?