System.Text.Json Polymorphism Type Discriminator Not Deserialized

[9ParsonsB]

Description

When deserializing a polymorphic type with TypeDiscriminatorPropertyName set to an existing Property name, that Property is no longer deserialized

Reproduction Steps

[JsonPolymorphic(TypeDiscriminatorPropertyName = "event")]
[JsonDerivedType(typeof(ChildEvent), "child")]
public abstract class EventBase
{
    [JsonPropertyName("timestamp")]
    public DateTimeOffset Timestamp { get; init; }
    
    [JsonPropertyName("event")]
    public string Event { get; set; }
}

public class ChildEvent : EventBase;

var json = """
           { "event": "child", "timestamp":"2024-05-20T12:36:10Z" }
           """;

var obj = JsonSerializer.Derserialize<EventBase>(json);

Console.WriteLine(obj.Event); // ""

// obj.Event = null;

Expected behavior

TypeDiscriminator Property is still deserialized

Actual behavior

TypeDiscriminator Property is always null/default

Regression?

No response

Known Workarounds

No response

Configuration

.NET 9.0.100-preview.3.24204.13
Arch Linux 6.8.9-arch1-1 x86-64

Other information

No response

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[eiriktsarpalis]

Duplicate of #72170 (comment). The runtime type of a property cannot be used to influence the discriminator which is a type-level mapping.