It is not a bug. What you view in the preview window is what is being encoded.
For the situation where you want a mirrored image for your own preview but encode a non-mirrored version, you can set up a drone scene. Create a new scene, and add a Scene Source. Select your original scene. Then, right-click on this, flip horizontally to get your mirror. Now, right-click on the scene and send it to a Projector (either as a full-screen instance or a window, depending on your use).
From there, you can go back to your original scene and you'll have the normal->stream / mirrored->self setup.
I really wish there was a way to just have an option for a mirrored projector, as this is something that is definitely useful. Also, this doesn't really allow for showing all scenes as you switch between them in that mirrored projector (it will only display the scene you've set up for this). The only way I know of to get around that is to use an NDI loopback, which is way more trouble than it's worth.
If anyone knows any better way to do that, I would love to know.