After
combining documents that contained track changes and reviewer
comments, heading numbers for levels five became blacked out.
This is definitely a bug. To resolve this, please do the following:
1. Put your cursor on the heading just right of the black box
2. Use the left arrow key on your keyboard to move left until the black box turns grey
3. Use the keyboard combination ctrl+shift+s, the dialog "Apply Styles" should appear
4. In this box, click "reapply"
1. Put your cursor on the heading just right of the black box
2. Use the left arrow key on your keyboard to move left until the black box turns grey
3. Use the keyboard combination ctrl+shift+s, the dialog "Apply Styles" should appear
4. In this box, click "reapply"
This fixed the issue for me. Good luck.
Reference: here
