It might be a performance problem then on my end. Did it switch scene when you press start? was it just the background itself, without the menu and player splash screen in the start? And do you hear dialogue beeps after you start?
I did some testing and got a similar situation but under different condition. If the player were to run past and attract all the enemy and they all start firing their projectiles, it would cause the game to lag out and it bugs out the camera to where it is just the background. But it does not happen for me at the start.
It could also be a problem with how I exported the resolution and window settings if you don't mind sharing it. I did windowed borderless at 1080p because it was easier for me to quickly work with, but forgot to switch it to a more friendly one for others. Other than that, I don't really know what to do. Last ditch effort would be trying to use a controller instead as it affects the camera a bit differently.