All users of Nexus S users are reporting troubles with Pure widgets when they applied the Jelly Bean update.
This issue is general to Android and affect a lot of paid applications.
This issue is known from Google and reported here
The problem seems to be related to default install location for this ROM (install on external memory, which is not possible for widgets).
A workaround is to use Titanium backup to make a backup and then uninstall the apps (using Titanium backup). Then use Titanium backup to re-install the apps and the widgets are here to stay
EDIT 11/10/2012 : the issue is now solve in Google Android code, but it will be published only in the next Android release (4.1.2 or 4.2).