We're sorry about the trouble with Lightroom. Would you mind telling us the exact version of Lightroom you're using along with the version of Windows you're working on? What export settings selecting while trying to export the images?
While trying to export the images, try selecting the option for Custom Settings & select the desired File Type (apart from Original + Settings). When you make a change to the selected File Type, you might see the original image flash for a second while Lightroom updates the preview of the edited image (this may or may not happen depending on your computers specification).
If the issue persists, try resetting the Preferences of Lightroom by pressing & holding the Shift + Alt keys while launching Lightroom. When you get the prompt to confirm resetting preferences, confirm & proceed.
Hi, I've got this problem too with Windows 10 21H1 19043.1466 and Lightroom 5.2.
I use the sky mask on a photo and that works like a charm, but when I want to export it (as JPG or any other format), the sky mask isn't applied on the exported photo. I have not found a way to export the photo with the sky mask applied to it.