Troubles exporting images in Lightroom Classic

This is most likely the following problem. You created a mask, but you did not really change the background. Instead you chose a mask overlay that changes the background, such as an overlay that makes it pure black. An overlay is not an edit however, that is a common misunderstanding. An overlay is just a way to show you the mask, so you know what will be affected and what will not be affected when you apply some changes using the sliders in the mask panel. And because you did not change any sliders, you did not edit the background, and so the exported image is still the same as the original image.