I am working an ASP.NET Core 6.0 project.

I wrote a query which is related to this property:

public List<Holiday> Holidays { get; set; }

It returns

"holidays": [ null ],

This happens after I change from JSON.NET to System.Text.Json.

So when using

builder.Services
       .AddControllers(options =>
                options.Filters.Add<ApiExceptionFilterAttribute>())
       .AddFluentValidation()
       .AddJsonOptions(options => options.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles);

But, when using

builder.Services
       .AddControllers(options =>
                options.Filters.Add<ApiExceptionFilterAttribute>())
                    .AddFluentValidation()
       .AddNewtonsoftJson(options => options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore);

the same code returns

holidays : []

Why does this happen?