In certain situations, the assignment feedback plugin edit pdf continuously calls 'pollconversions' in /mod/assign/feedback/editpdf/ajax.php.

I have not been able to identify the steps to reproduce the issue.

However, I have identified where the issue is in the code.

https://github.com/moodle/moodle/blob/8e9e9a5f7e425b8f7f2b3d0702c3403fb9b823aa/mod/assign/feedback/editpdf/ajax.php#L84-L87

The combineddocument  response status never gets refreshed after get_combined_pdf_for_attempt() so is always stuck in ready status and also will never make it to the block of code that generates page images etc.