Field Security issue copying CRM production instance to sandbox

We recently ran into an issue restoring the a production CRM instance to the corresponding sandbox instance. Although the field security profiles appeared to be copied correctly, the secured fields were not accessible, even with the System Administrator security role.

The workaround is to:

  1. Toggle field security off on the field and save it.
  2. Toggle field security back on and save it.

Do this for each secured field. Then rebuild the Field Security profiles.