Before testing your file:
> using the stream got the leak
I asked if you are sure you are disposing your own stream like this?
using (var imageTask = new ImageTask(stream, null, false, _configuration))
if (imageTask.Info != null)
Because you say leak does not happen with file path, so probably the leak comes from your own "not disposed" stream. ImageTask will not dispose your stream internally. In .NET Framework as best practice, the classes will not dispose/close your input stream. If you open a stream, you need to ensure closing it after used.
> as mentioned above even if i don't create thumbnails just call the Info property leak occur for webp files
FYI, accessing Info property also means reading the image file from stream.