XF 1.5 After vB import, members aren't taken to unread post for new posts


Active member
After vB3 import (and then subsequent xenforo new posts), members aren't taken to an /unread link to get to the last post of threads with new posts made in xenforo, but instead to the first post of the thread. Clicking the mark read link solves the issue for future posts. Is this a known issue? Why does it occur?