Sorry for not getting to this sooner - I've been out on leave.
I'm really surprised that this issue has arisen. If the Spring lifecycle no longer applied to plugin components, this would break dozens of plugins and probably some of our tests too. We haven't heard anything about this.
I'll need to investigate and write a simpler test case for this to confirm the issue before looking at a fix. This should be fixed in Confluence, not in the plugin.
Debug output for 1.4 will show the plugin gets loaded. Nevertheless, when a new user logs in there are NO log entries and his groups do not get added. Same thing when running migrate-external-memberships.action. With Joes modification however we do see log entries as well as groups getting added.