Security and privacy

Educated guess ....

If a resource in Moodle has been uploaded to Moodle, the software uses the $CFG->wwwroot URL (ie, the http:// )  amd records that in the DB.  If, later on, one changes the config file to https://, that doesn't change how the link is recorded in the DB.  Moodle doesn't use relative addressing.

So check DB for those URL's ... there is a search and replace tool in Moodle but think I'd inspect the tables first and do only one resource change then check it, before using the s&r tool.
'spirit of sharing'. Ken