The link is showing for posts, however the box that opens has no detail:
The user profile in Admin has this for that user:
No IP logs were found for the requested user.
However when I check back on my old vbulletin before importing to xenforo there is an IP address for this user.
NB: Other users I checked do have IP addresses which survived the import (however for all of them, the box from the link under posts shows blank as above, but perhaps I should report that as a separate issue)