Hi Severin,
I've recently found that this enrolment method has an unexpected behaviour on Moodle 3.7 (I would say it's a bug, but I'm not sure): when you add a meta-course enrolment method to a course and then disable it, every time the scheduled task is executed it enrols all the users of that enrolment method and then unenrols then again. That behaviour makes the execution of the task longer than expected and inserts a lot of non-meaningful registers to the log (and then it leaves the enrolments just the same).
If you find any recently created/modified disabled meta-enrolment methods on mdl_enrol it may be the reason of your issue.
Cheers.