I have 500+ pictures that I edited in Lightroom classic and for all of them I used a background mask. the problem is that when I export the photos, around 100+ of them are without the edit that I made on all of them, the background mask is missing. I tried to export them again and will the process was ongoing the mask just disapeared without clicking on the pictures or doing anything.
Can you explain in a bit more detail what 'background mask' these images had? The reason I am asking is that we often get a question like this, and then it turns out that people confuse the mask overlay with the mask effect. So they create a mask, see the overlay and like how that looks, and then think this is how they edited the image. On export, the mask seems to be missing because the overlay is not an edit, it's just a view to show you what is masked and what is not masked.