If the above suggestions don't work you can also try the following.

1. Try changing the version of proton the game uses. You can do this on your deck by going to Not For Broadcast. Then select the Settings icon (The cog on the right side of the screen) > Then properties > Then Compatibility.
Have a play with a few different versions and see if that does the trick.

2. If that doesn't work you can try the "alternatevideo" branch which will download an alternative version of the videos. You can access this by pressing the Settings button from before. > Then properties > Betas > Then in the drop down select "alternatevideo"
You might find that your Steam Deck doesn't have enough space in which case you will need to uninstall Not For Broadcast and then re-install it with the "alternatevideo" branch selected.