I just tested this Sam and it worked for me, the Learner role was assigned.

From looking at the code it should work as long as the Learning Plan enrolment plugin is properly configured -  can you double-check Site Admin -> Plugins -> Enrolments -> Manage enrol plugins?

1) Make sure that Learning Plan enrolment plugin is enabled

2) Make sure on the settings for the plugin that the Default role is selected and you have clicked "Save changes"

3) Maybe make sure the Learning Plan enrolment plugin is one of the first plugins checked - in the course, Course Administration -> Users -> Enrolment Methods move "Learning Plan" up to near the top of the list especially ahead of Guest access (if enabled).