When you first load up the game with a mod enabled it displays Tag not found followed by a cut off tag name where the class should be on on all the characters on the character selection screen using a new mastery(with the occasional ? for single class characters).
When you go into the game the character panel displays the class names appropriately. Then when go back to the character selection it also displays the class names appropriately.
All of my custom mastery tags are in a separate file from the original tag_skills.txt. It seems when you first load it only references the original file but not any additional ones added by a mod. I can only assume this is some kind of bug with how the game loads additional tag files but I was just wondering if anyone else can confirm this or has some work around besides using the original .txt file.