Remove Background Not Displaying Properly in Downloaded MP4

I've made out a animation for an Instagram Reel (based on some others I've created in the past that worked as expected) where I have images animate in. I've used both the "Darken" Effect and "Remove Background". In the preview when I play the animation it looks exactly as intended, but when I export it as a 4K MP4, there is a gray background where the background was removed in the preview. I was hoping to share this as an IG reel this morning, but then noticed the issue. If someone is able to offer a solution as soon as possible, I'd greatly appreciate it 🙂