Has been an issue for quite some time. Noticed it was mentioned in another issue briefly before but it's a constant issue currently that any time a check the tooltip for an item I don't own or that hasn't been looked at in that game session, the UI must be /reload before the correct information appears. I have waited several minutes after looking at the tooltip to give it time to be "cached" (not sure what exactly the process is), but it often doesn't load properly even after that long a time period.

Hopefully a workaround is possible. I imagine a large number of players who aren't as "savvy" have probably passed on items without realizing that certain stats are not calculated on the tooltip due to this error.