for some reason when a user self enrols in a course it takes a while for that course to appear in the list of courses for the user in the app (we detected that it can take up to 20 seconds). That's something that's caused by Moodle web.
When self enrolling in the app (using the core method), we usually show a loading modal and wait for the course to be displayed in the list of courses of the user before letting him continue using the app. You say you're using a plugin for the enrolment, I guess with that plugin the enrolment process is different and doesn't trigger this loading modal. Can you please try to wait a few seconds and then do a Pull To Refresh in the list of courses to see if the course appears in there after the refresh?