Upon investigating this issue again by comparing two websites using different themes and comparable (same) plugins I found out that another plugin was unexpectedly interfering with the way a page was build for display.

The plugin Ovation Elements is signalled as being compatible with WordPress version 6.8.2 in the Plugin selection screen, but in its description it says compatibility has only been tested up to WordPress 6.6.1. So this plugin is not compatible with my version of WordPress