Version (25.6.23.2432) headers not pinned on top of the pages anymore
No custom themes used
And the image you provided shows exactly what i mean
As you can tell from your image headers don’t appear in second page
This is a limitation of chrome-based browsers. This limits the header to no more than 250px.
You may have not noticed this before using Internal PDF Generator feature which is now obsolete.
The only solution is to edit the theme and resize your table header to not exceed 250px.