Bug - Extra Prefix

Steps to Reproduce

Create a new blank doucment
Insert the text "My heading 1"
Format this text with the built-in "Heading 1" style
Press Enter twice, Insert the text "My heading 2"
Format this text with the built-in "Heading 2" style

This problem only occurs when you are using a multi-level list for your headings.
Put your cursor at the end of "My heading 1" text.
On the Home tab, click on the "Multilevel List" drop-down and select the item under "Current List".
Your "My heading 1" text will now be displayed with a "1." field in front of it.

Put your cursor at the end of "My heading 2".
On the Home tab, click on the "Multilevel List" drop-down and select "Current List"
Expand "Change List Level" and select level 2.
Your "My heading 2" text will now be displayed with a "a." field in front of it.

Move your cursor to the last paragraph.
On the References tab, click on "Cross-reference".
You should see both these headings appear in the list.
Select the 2nd item and press Insert.
You will see an "a" inserted as your cross-reference.

Move your cursor to the first paragraph.
On the References tab, click on "Cross-reference".
Select the 2nd item and press Insert.
You will see an "1.a" inserted as your cross-reference.

Solution

Important

Word does not use a multi-level list for its "default" heading styles so we need to create a new multi-level list.