Fix null return condition in NullableConverter<T>.OnTryRead

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Union/JsonUnionConverter.cs

@@ -38,7 +38,7 @@ internal override bool OnTryRead(ref Utf8JsonReader reader, Type typeToConvert,
                 ThrowHelper.ThrowJsonException_UnionCannotCreateValue(typeToConvert);
             }
 
-            if (reader.TokenType is JsonTokenType.Null)
+            if (reader.TokenType is JsonTokenType.Null && reader.CurrentDepth == state.Current.OriginalDepth)
             {
                 Type? nullableCaseType = typeInfo.UnionNullableCaseType;
                 if (nullableCaseType is null)

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/NullableConverter.cs

@@ -29,7 +29,7 @@ public NullableConverter(JsonConverter<T> elementConverter)
 
         internal override bool OnTryRead(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options, scoped ref ReadStack state, out T? value)
         {
-            if (!state.IsContinuation && reader.TokenType == JsonTokenType.Null)
+            if (reader.TokenType == JsonTokenType.Null && reader.CurrentDepth == state.Current.OriginalDepth)
             {
                 value = null;
                 return true;

src/libraries/System.Text.Json/tests/Common/UnionTests.cs

@@ -1026,6 +1026,22 @@ public async Task UnionWithDefaultTokenClassification_RoundTripsObjectCaseAsProp
             Assert.Equal(new[] { 6, 7, 8 }, payload.Values);
         }
 
+        [Fact]
+        public async Task UnionWithDefaultTokenClassification_ObjectCaseWithNullPropertyAsPropertyWithSmallBuffer()
+        {
+            JsonSerializerOptions options = CreateSmallBufferOptions();
+
+            string json = """{"Union":{"Name":null,"Values":[6,7,8]},"After":42}""";
+            UnionContainer? result = await Serializer.DeserializeWrapper<UnionContainer>(json, options);
+
+            Assert.NotNull(result);
+            Assert.Equal(42, result!.After);
+            Assert.NotNull(result.Union);
+            PayloadCase payload = Assert.IsType<PayloadCase>(GetUnionValue(result.Union!));
+            Assert.Null(payload.Name);
+            Assert.Equal(new[] { 6, 7, 8 }, payload.Values);
+        }
+
         [Fact]
         public async Task UnionWithDefaultTokenClassification_RoundTripsNullableScalarCaseAsPropertyWithSmallBuffer()
         {

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/NullableTests.cs

@@ -4,6 +4,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.IO;
 using Xunit;
 
 namespace System.Text.Json.Serialization.Tests
@@ -489,5 +490,29 @@ public struct RecursiveNullableStruct
         {
             public RecursiveNullableStruct?[] Next { get; set; } 
         }
+
+        [Fact]
+        public static void NullableStructProperty_StreamWithContinuationAfterNullToken()
+        {
+            string json = """{"Value":{"Total":1,"a":null,"b":0}}""";
+            byte[] bytes = Encoding.UTF8.GetBytes(json);
+            var options = new JsonSerializerOptions { DefaultBufferSize = 32 };
+
+            using var stream = new MemoryStream(bytes);
+            var fromStream = JsonSerializer.Deserialize<MyPoco>(stream, options)!;
+            Assert.NotNull(fromStream);
+            Assert.NotNull(fromStream.Value);
+            Assert.Equal(1, fromStream.Value.Value.Total);
+        }
+
+        public class MyPoco
+        {
+            public MyStruct? Value { get; set; }
+        }
+
+        public struct MyStruct
+        {
+            public int Total { get; set; }
+        }
     }
 }