AE video goes black when using pen or rectangle

Hi folks, B here.

After trying to solve what may be a glitch or simple oversight I finally need to reach out to the hive mind. I am experiencing a problem in After Effects with a single MOV video 

My steps 

1. Import the .MOV into a new composition.

2. Select the video layer in the timeline.

3. Use the Pen Tool or Rectangle Tool to draw a mask around a small bright blob in the footage.

4. In the timeline under Mask 1:

    

   • Mask Mode = Add, no choice really works properly

   • Inverted = unchecked

   • Feather = 2–5 px

    

5. Observation: The video blacks out outside the mask, leaving only the masked area visible.

6. Open Content-Aware Fill → Fill Method = Object → Range = Work Area → Generate Fill Layer.

    

   • The issue persists: original video outside the mask remains black.

    

• Tried resetting workspace and creating a fresh composition.

• Mask path appears correctly under the video layer.

• Tried Pen/Rectangle tools with RotoBezier on/off.

• Timeline “None” dropdown ignored — no effect.

When I have changed to a smoother codec in Media Encoder the video embeds two black areas to the left and right, making video useless.

Can you advise why the video blacks out when applying a mask to the video layer, and how to fix it?

Thank you.