System Information
Operating system: Windows 10 Home
Graphics card: GTX 1660 Super

Blender Version
Broken: 3.0.0 and 3.1.0 (December 5th 2022 daily build)
Worked: 2.93.5

Short description of error
FFmpeg renders are corrupted and completely useless at least in 1080x1350 (portrait mobile videos). Left-hand side gets some of the right-hand side X dimensions visual subject matter baked into the video when displayed from the view animation panel (ctrl f11) inside Blender. Looks normal albeit laggy in the output folder. But furthermore if you render two of these and then start to edit those inside Blender as video clips, the effects get baked into the final render.

Exact steps for others to reproduce the error
Use default project. Select active camera and set resolution X 1080 and Y 1350 100%. Animate cameras default X 7.35889 m to X Location to 4 m so the cube is off-screen. File format: FFmpeg Video, Encoding h264 in mp4. I used High quality but probably irrelevant. Render animation. Render - view animation. When the cube gets to right-hand side you can clearly see the cube on the left side.