extend the test and fix it in the BufferedStream as well

src/libraries/System.IO.FileSystem/tests/FileStream/Read.cs

@@ -41,7 +41,16 @@ public void NoInt32OverflowInTheBufferingLogic()
                 Assert.Equal(buffer.Length, stream.Read(buffer));
                 Assert.Equal(data1, buffer);
 
-                Array.Clear(buffer);
+                stream.Seek(positon2, SeekOrigin.Begin);
+                Assert.Equal(buffer.Length, stream.Read(buffer));
+                Assert.Equal(data2, buffer);
+            }
+
+            using (var stream = new BufferedStream(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.None, bufferSize: 0)))
+            {
+                stream.Seek(positon1, SeekOrigin.Begin);
+                Assert.Equal(buffer.Length, stream.Read(buffer));
+                Assert.Equal(data1, buffer);
 
                 stream.Seek(positon2, SeekOrigin.Begin);
                 Assert.Equal(buffer.Length, stream.Read(buffer));

src/libraries/System.Private.CoreLib/src/System/IO/BufferedStream.cs

@@ -1222,11 +1222,12 @@ public override long Seek(long offset, SeekOrigin origin)
             // Otherwise we will throw away the buffer. This can only happen on read, as we flushed write data above.
 
             // The offset of the new/updated seek pointer within _buffer:
-            _readPos = (int)(newPos - (oldPos - _readPos));
+            long readPos = (newPos - (oldPos - _readPos));
 
             // If the offset of the updated seek pointer in the buffer is still legal, then we can keep using the buffer:
-            if (0 <= _readPos && _readPos < _readLen)
+            if (0 <= readPos && readPos < _readLen)
             {
+                _readPos = (int)readPos;
                 // Adjust the seek pointer of the underlying stream to reflect the amount of useful bytes in the read buffer:
                 _stream.Seek(_readLen - _readPos, SeekOrigin.Current);
             }