Where are you exporting the Files?
Please reset Lightroom Preferences
First Backup your current catalog Adobe Photoshop Lightroom Help | Back up a catalog
Once done Reset Preferences .Refer Resetting Preferences in Lightroom CC/6 | Lightroom Blog
Let us know if that helps
There are known issues, which not really are fixed with 6.2.1: https://helpx.adobe.com/lightroom/kb/lightroom-and-el-capitan---mac-os-10-11.html