Editing issue removes due date

When a user without administration rights to a project, or at least without rights to edit the due date for an issue, edits the issue, the due date disappears. The problem does not happen if the user has the rights to alter the due date.

To replicate this issue, we created a mock issue for a user without rights to alter the due date. Every time the user changed the time estimate or any other field in the issue, the due date was removed.

We also tried giving the same user permission to schedule issues for the project in question and the due date was maintained.