Unable to use preview to test site - user not logged in

Help needed. I have a members area that requires the user to be logged in (the dataset is filtered on user is owner). Up until a couple of hours ago I was able to preview the site but I am now getting an error. This appears to be because the filtered dataset is empty. In preview the admin should be logged in and admins name does appear on the screen but when I checked the user details it reports that the user is not logged in and role is visitor. The live site is working normally even after saving and republishing. Any Ideas?

Cant use preview when creating membership sites now. Feature request added for Preview As… and let you choose which user to preview as.

Has this changed within the last 12 hours?