Roles and permissions

We are using Moodle 1.9.3 (Build: 20081015)

Added three custom user profile fields for some important ID numbers we use, marking them all as "Locked" and "Visible to user". This we understood would allow only admin to add or edit these fields, but let the user see them. All great.

But recently we noticed that some of the data in these fields was "disappearing". Careful investigation and testing proved that if a student (user) edited their profile (even just open it and "save"), even that action deleted the content of those fields. Only by setting the fields to "Not visible" could we avoid the problem - but thats not ideal as now the student can't see them at all.

Has anyone else found this problem, and hopefully a fix? Did try searching here, but found no similar discussion.

Nigel