Hello. I suspect it is not a bug but the fact that the dashboard is not dependent on a role, so Moodle doesn't know that a person is a teacher or student when the dashboard page is displayed. You could probably get around it by creating a new custom role (eg Dashboard teacher) and giving that role in the system context to all your teachers and then setting the tour on the dashboard to display for 'Dashboard' teachers... it depends on how important you feel it is that teachers should have their own user tour different from students.
Thank you Mary for your insight into this issue. Your assessment of the issue makes perfect sense and you've provided a good solution.