Ensure the async reader state resets the BOM offset in every AdvanceB…

…uffer() call.

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/ReadBufferState.cs

@@ -122,7 +122,6 @@ public void AdvanceBuffer(int bytesConsumed)
                     // Copy the unprocessed data to the new buffer while shifting the processed bytes.
                     Buffer.BlockCopy(oldBuffer, _offset + bytesConsumed, newBuffer, 0, _count);
                     _buffer = newBuffer;
-                    _offset = 0;
                     _maxCount = _count;
 
                     // Clear and return the old buffer
@@ -133,9 +132,10 @@ public void AdvanceBuffer(int bytesConsumed)
                 {
                     // Shift the processed bytes to the beginning of buffer to make more room.
                     Buffer.BlockCopy(_buffer, _offset + bytesConsumed, _buffer, 0, _count);
-                    _offset = 0;
                 }
             }
+
+            _offset = 0;
         }
 
         private void ProcessReadBytes()

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

@@ -235,6 +235,31 @@ public static void InvalidJsonShouldFailAtAnyPosition_Sequence(
             Assert.Equal(expectedFailure.Column, ex.BytePositionInLine);
         }
 
+        [Fact]
+        public static async Task BomHandlingRegressionTest()
+        {
+            byte[] utf8Bom = Encoding.UTF8.GetPreamble();
+            byte[] json = """{ "Value" :                    "Hello" }"""u8.ToArray();
+
+            using var stream = new MemoryStream();
+            stream.Write(utf8Bom, 0, utf8Bom.Length);
+            stream.Write(json, 0, json.Length);
+            stream.Position = 0;
+
+            var options = new JsonSerializerOptions
+            {
+                DefaultBufferSize = 32
+            };
+
+            Test result = await JsonSerializer.DeserializeAsync<Test>(stream, options);
+            Assert.Equal("Hello", result.Value);
+        }
+
+        private class Test
+        {
+            public string Value { get; set; }
+        }
+
         private class Chunk : ReadOnlySequenceSegment<byte>
         {
             public Chunk(string json, int firstSegmentLength)