EF Core 5 - Automatic Many to Many throws "property ... cannot be assigned a temporary value"

Question

We have a very large context was generated using scaffolding a couple years ago, back in .Net Core 3. We are already running .NET 6 and I wanted to use the automatic Many to Many relationship creation that was released with EF Core 5 for a new feature, user alerts. The thing is I configured and created a migration and everything seems to be in place but when trying to create an alert for multiple users and link them with the many-to-many relationship an exception is thrown:

The property 'AlertUser (Dictionary<string, object>).AlertsId' cannot be assigned a temporary value.
Temporary values can only be assigned to properties configured to use store-generated values.

I can't find anything about it googling around so I know it can be something very specific to our model but everything seems fine:

Alert.cs

[Table("Alert")]
public class Alert
{
  [Key]
  [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
  [Column("AlertID")]
  public long Id { get; set; }

  public string MessageText { get; set; }

  public ICollection<User> Users { get; set; }
}

User.cs

[Table("Users")]
public class User
{
  [Column("UserID")]
  [DatabaseGenerated(DatabaseGeneratedOption.None)]
  public override long Id { get; set; }

  [Column("Username")]
  [Required]
  [StringLength(255)]
  [NonUnicode]
  public override string UserName { get; set; }

  public ICollection<Alert> Alerts { get; set; } = new HashSet<Alert>();
}

Basically a user can have many alerts and an alert can point to many users. To create an alert, I followed the official MSFT docs and several tutorials, I thought I could do something like:

var users = await _dbContext.Users
  .Where(u => userIds.Contains(u.Id))
  .ToListAsync();

  var alert = new Alert
  {
    MessageText = message,
    Users = users
  };

  await _dbContext.AddAsync(alert);
  await _dbContext.SaveChangesAsync();

But the debugger never reaches SaveChanges and throws the exception in the await AddAsync. I've been trying a couple things that did work like creating the Alert and then updating the users with the alerts for each one, so I think it might be something with the way SQLServer is generating the keys or something like that. I wanted to know if someone else did face this error and what can we be doing wrong.

Also, here's the generated migration:

migrationBuilder.CreateTable(
  name: "AlertUser",
  columns: table => new
  {
    AlertsId = table.Column<long>(type: "bigint", nullable: false),
    UsersId = table.Column<long>(type: "bigint", nullable: false)
  },
  constraints: table =>
  {
    table.PrimaryKey("PK_AlertUser", x => new { x.AlertsId, x.UsersId });
    table.ForeignKey(
      name: "FK_AlertUser_Alert_AlertsId",
      column: x => x.AlertsId,
      principalTable: "Alert",
      principalColumn: "AlertID",
      onDelete: ReferentialAction.Cascade);
    table.ForeignKey(
      name: "FK_AlertUser_Users_UsersId",
      column: x => x.UsersId,
      principalTable: "Users",
      principalColumn: "UserID",
      onDelete: ReferentialAction.Cascade);
    });

    migrationBuilder.CreateIndex(
      name: "IX_AlertUser_UsersId",
      table: "AlertUser",
      column: "UsersId");