Why do you think this is happening

I have created an animation for a client, but when I export it it shows different than when in after effect, have a look at the screen recording and the exported video...

I am talking about the black background shown at the beginning with the mask, I have added a solid grey background in my project, 

I really don't know what to try anymore

I am attaching the screen recording from AE the exported video and also the project.
Find the project folder https://1drv.ms/f/s!AnzCEK8poKX0ktlw2BBqS6enj8iZ9Q?e=VVwlMO