IMS Common Cartridge backup not available

I'm running Moodle 4.5.6+ (Build: 20250829) on a fully-patched RHEL9 with PHP 8.2.28.

In a course, I go to More -> Course re-use -> Backup and I don't get offered any option to export the course in IMSCC format. The checkbox just doesn't appear.

I've got all the XML, XSL, DOM, libXML options enabled in PHP, just re-checked them in Server -> PHPinfo. In 4.5.6 there aren't any Development -> Experimental settings related to IMSCC. I've tried switching theme back to Boost in case Boost Union was causing a problem: no change.

Any ideas why I'm not even being offered the option of backing up to an IMS CC file?
Googling has produced nothing useful except the checks I've already made above.

Thanks!
Jules.