Pre and post export doesn’t look the same, black gets really white grainy

Hi!

I taking live photos with my canon 5D, so often it's pretty dark.

when I edit the photos in lightroom mobile, photos looks how I like, and black is böack and so on.

when I export the photos, the saved version looks very grainy, everything that is black looks like white grain.

I've teied with all differebt export setting but looks the same.

sme thing happends if start zoomin thw photo, first it looks normal and then it just changes to this where black isn't black anymore.

I'm pretty new with this all, so could there be something basic stuff that I am doing wrong?

I like how the photo looks before I'm exportig but not happy st all after the export.

original photos are raw photos.

thanks already everybody!

-Bobby