Why the special behaviour when FK identity is zero? - entity-framework

Question

I have three tables:
UserProfiles (UserId is the primary key
UserId Email
------ ------------
9032 bill#any.com
WebpagesRoles (RoleId is the primary key)
RoleId RoleName
------ ------------------
0 Site User
56 Admin
(Note the zero RoleId - that's the tricky bit.)
And WebpagesUsersInRoles, where both columns are foreign keys to the other tables.
I want to add these rows to WebpagesUsersInRoles
UserId RoleId
------ ------
9032 0
9032 56
My C#, EFCore 3.1 application has been deoptimised to aid investigation
foreach (var uirToAdd in uirsToAdd)
{
_context.WebpagesUsersInRoles.Add(uirToAdd);
// var role = _context.WebpagesRoles.FirstOrDefault(r => r.RoleId == uirToAdd.RoleId);
// uirToAdd.Role = role;
await _context.SaveChangesAsync(CancellationToken.None);
}
The code snippet above can write the "Admin" row where RoleId is non-zero, but it can't write the "SiteUser" row where RoleId is zero. It fails on the SaveChangesAsync with the message 'The INSERT statement conflicted with the FOREIGN KEY constraint "fk_RoleId"'.
However, if I uncomment those two lines, it works fine.
Why?
Note, we are not using WebSecurity, just the tables, and so a command like roles.AddUserToRole("admin1", "Admin") is not appropriate. In any case, I'm just as curious about what's causing the problem as in finding a solution.
Edit
For reference, here's the actual model, including some parts I left out of the question for brevity:
public partial class WebpagesRole
{
public WebpagesRole()
{
ActionDropdownRoles = new HashSet<ActionDropdownRole>();
ActionTypeRoles = new HashSet<ActionTypeRole>();
ClientTypeRoles = new HashSet<ClientTypeRole>();
ScimUserGroupRoles = new HashSet<ScimUserGroupRole>();
WebpagesUsersInRoles = new HashSet<WebpagesUsersInRole>();
}
public int RoleId { get; set; }
public string RoleName { get; set; }
public string Description { get; set; }
public virtual ICollection<ActionDropdownRole> ActionDropdownRoles { get; set; }
public virtual ICollection<ActionTypeRole> ActionTypeRoles { get; set; }
public virtual ICollection<ClientTypeRole> ClientTypeRoles { get; set; }
public virtual ICollection<ScimUserGroupRole> ScimUserGroupRoles { get; set; }
public virtual ICollection<WebpagesUsersInRole> WebpagesUsersInRoles { get; set; }
}