Figured it out. The problem was with search-guard-ssl plugin. I was using the version 2.3.3.15 and this is apparently a bug in that version. I tried the latest version 2.3.3.21 and I don't see any outstanding tasks (except the "task list" task itself).
Basically search-guard-ssl-2.3.3.15 was not calling the unregisterTask() method on tasks after consuming them. So the tasks still remained in the list and being shown as still running.