Summary: When using HarfBuzz and the World Ready Composer, automatic fields such as the current page number are truncated to their first character when exporting to PDF.

Steps to reproduce the problem:
1. Enable HarfBuzz rendering mode. (Script: app.textPreferences.shapeIndicAndLatinWithHarbuzz = true;)
2. Create a new document.
3. Add a text frame.
4. In the text frame, insert a marker for the current page number. (Type > Insert Special character > Markers > Current page number.)
5. Set the paragraph justification to use Adobe World Ready Composer.
6. Change the page numbering in some way so that the page number will have more than one character. e.g. Layout > Numbering & Section Options > Start at page 8, using roman numerals.
7. Create a few additional pages, likewise each displaying the page number. (e.g. viii, ix, x, xi...)
8. Export to PDF.

Desired behavior: Page numbers appear as in InDesign. e.g. vii, ix, x, xi.

Actual behavior: Only the first character of each page number appears. e.g. v, i, x, x

Versions of InDesign tested: This happens in 17.4. It does not happen in 16.4.3.