The in-memory implementation does not behave the same as a real SQL server. There is a statement made in this article: https://learn.microsoft.com/en-us/ef/core/testing/#involving-the-database-or-not which suggests:
The in-memory provider will not behave like your real database in many important ways. Some features cannot be tested with it at all (e.g. transactions, raw SQL..), while other features may behave differently than your production database (e.g. case-sensitivity in queries). While in-memory can work for simple, constrained query scenarios, it is highly limited and we discourage its use.

Because table TodoItems not has column 'Secret'.
I think table was define before.
you can use migrate to add new column or ignore mapping that column.

namespace TodoApi.Models
{
    public class TodoItem
    {
        public long Id { get; set; }
        public string? Name { get; set; }
        public bool IsComplete { get; set; }
        [NotMapped]
        public string? Secret { get; set; }
    }
 }