The way our import worked, every single account had “Last Seen” to the day the import ran.
…
All 37000 of them.
If you haven’t already gone live, look really carefully at all your defaults. If you have, for example, a “help” or an "Announcement’s category that you want everyone to be watching by default, then set that in settings AND set it for all the accounts you’ve imported (or verify that they are set correctly).