Wrong heading numbers in PDF export when setting numbers on h1

Steps to reproduce:

Create a page

= Heading 1 =

= Heading 2 =

(% data-numbered-headings-start="5" style="--numbered-headings-start: 4;" %)
= Heading 5 =

= Heading 6 =

Enable heading numbering and export it to PDF using client-side PDF export in Firefox.

Expected result:

Heading 5 has number 5 in the export.

Actual result:

Heading 5 has number 4 in the export, "Heading 6" has number 5.

I also couldn't reproduce this issue when setting the number on a subheading.