If a teacher manually adds some activity links as relative urls (e.g. '/mod/forum/view.php?id=4852581') to any text in the course (e.g. section description), then these links will be pointing to an old activities after backup and restore process.
It looks like most of the activities expect links to be full URLs to be able to encode/decode them on backup/restore.
Also as per Damyon Wiese comment in the dev telegram channel "links in content that do not include the domain name won't work when the content is served from a different location (e.g. in the mobile app)"
So if we know that moodle behaves this way we probably should somehow prevent users to use relative urls or at least warn somehow... Or it could be different solution.