Upgrading to 4.2 or above loses all labels on page templates

Symptoms

If there are any labels on templates on a pre-4.2 version of Confluence, and it is upgraded to 4.2 or above:

1. The labels no longer appear on the templates
2. The labels still exist in the database in the "labels" column of the pagetemplates table
3. If the version is 4.2.x, then every time the template is saved, the labels which used to exist are added to the template, and are unable to be removed again. This repeats for every time the template is saved. This doesn't happen if Confluence has been upgraded to 4.3 or above.

Steps to Reproduce

1. On any version of Confluence before 4.2, add a global page template
2. Add a label on the template
3. Upgrade to 4.2
4. Observe the template labels are missing

Manual Workaround

You can get a list of the templates that used to exist by running an SQL query:

select templatename, labels from pagetemplates where labels is not null;

Then re-add the labels to these templates manually.