Editing CiviContribute membership page loses price set selection

If you have entered a CiviContribute page and used a price set on the Memberships section under "Membership Price Set" on that page, when you go back to re-edit the Memberships section the selected price set is still shown as selected.

However, when you submit the form and go to check the page itself you will find the price set has disappeared.

When you go back to to the Memberships section for the page you will find that the price set selection has now disappeared. If you re-select it and press submit again it will work again (until the next time you edit the page!).

This is for CCRM 4.2 under Drupal 6.