Table of Contents macro doesn't work when using the Legacy editor

Issue Summary

The Table of Contents macro is no longer working when using the Legacy editor. It appears blank after the page is published.

Steps to Reproduce

1. Create a page using the Legacy template
2. Add headings to the page.
3. Add the Table of Contents macro in the beginning of the page.
4. Publish the page.

Expected Results

The Table of Contents macro should be displayed.

Actual Results

The Table of Contents macro appears blank.

Workaround

Users can convert the page into the Fabric editor (current one).