Upon scanning, the plugin only checks the revision of the main directory of the trunk and branches, see partial log here:

Checking directory /Jenkins/foo_main_project/branches@34

Checking candidate branch /Jenkins/foo_main_project/trunk@95
      ‘Jenkinsfile’ found
Met criteria
No changes detected: trunk (still at 95)

When commiting changes to externals within some subdirectory, the revision number of the trunk or the branch will not change. Thus it is necessary to check the revision numbers of trunk, branches and all externals within those.