You can fix this in 3 steps.
- In the Sources box in the lower left corner, click "+" and select a type. If you're on a PC, you'll select Game Capture. If you're on a Mac, you'll select Window Capture. Click OK to confirm.
- As you select from the Window list, select your Minecraft window.
- Back to the Sources box, make sure you move the Game Capture or Window Capture to the top of the list so that it's the default for all future recordings.