System.Text.Json ruins 'CustomField' property on saving backlog item

[AKlaus]

Sometimes on saving a backlog item a weird property mutation happens.

A CustomFields property

"CustomFields": {
    "33-A": "https://github.com/dotnet/aspnetcore/issues/24068"
}

gets mutated to

"CustomFields": {
    "33-A": {
        "$type": "System.Text.Json.JsonElement, System.Text.Json",
        "TokenType": "None",
        "ValueKind": "Undefined",
        "DebuggerDisplay": "ValueKind = Undefined : \"\""
    }
}

That's causing pain on reading it:

Operation is not valid due to the current state of the object.
at System.Text.Json.JsonElement.CheckValidInstance()
at System.Text.Json.JsonElement.WriteTo(Utf8JsonWriter writer) 
at System.Text.Json.Serialization.Converters.JsonElementConverter.Write(Utf8JsonWriter writer, JsonElement value, JsonSerializerOptions options) 
at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.Serialization.JsonConverter`1.TryWriteAsObject(Utf8JsonWriter writer, Object value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.Serialization.Converters.IDictionaryOfTKeyTValueConverter`3.OnWriteResume(Utf8JsonWriter writer, TCollection value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.Serialization.Converters.DictionaryDefaultConverter`3.OnTryWrite(Utf8JsonWriter writer, TCollection dictionary, JsonSerializerOptions options, WriteStack& state)
at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.JsonPropertyInfo`1.GetMemberAndWriteJson(Object obj, WriteStack& state, Utf8JsonWriter writer) 
at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryWrite(Utf8JsonWriter writer, T value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.Serialization.JsonConverter`1.WriteCore(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.Serialization.JsonConverter`1.WriteCoreAsObject(Utf8JsonWriter writer, Object value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.JsonSerializer.WriteCore[TValue](JsonConverter jsonConverter, Utf8JsonWriter writer, TValue& value, JsonSerializerOptions options, WriteStack& state) 
at System.Text.Json.JsonSerializer.WriteAsyncCore[TValue](Stream utf8Json, TValue value, Type inputType, JsonSerializerOptions options, CancellationToken cancellationToken) 
at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonOutputFormatter.WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding) 
at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Logged|21_0(ResourceInvoker invoker, IActionResult result) 
at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.g__Awaited|29_0