Specific exclude regex in ToC is not working in view page mode, but is in preview

Summary

Not always does the Table of Context excludes correctly the 'exclude regex' from the search. For example, the regex '^(?![\s\t\w]*$).+}', which should eliminate titles only with spaces, tabs, etc. useful to remove unnecessary titles, works in 'preview' view, but not after we save the page.

Steps to Reproduce

1. Create 4 sections in a page
2. Add in 3 'h1' headings with text, and h2 in another
3. Add a ToC in the other section and add an 'almost empty' h1 heading (with only a space in the title) in this section
   • Make sure to select the space and see if it's heading 1
4. Add remove regex: '^(?![\s\t\w]*$).+'
5. Check if the empty header is included in ToC

Expected Results

It does not include the heading with a single space in preview mode or after saving the page.

Actual Results

It's excluded from the preview mode, but not when we save the page.

Workaround

None so far.