Cannot cancel partially paid contributions

This is a two-part problem.

1. Event payments that are less than the event amount are automatically saved as contributions with "partially paid" status even if the contribution status dropdown is set to "complete" when the event registration is added.

The event registration status near the top changes to "partially paid" but it's far from obvious unless you know to look for it.
The contribution status remains "complete" but when the registration is saved, the contribution is recorded as partially paid.

2. The partial payment cannot be cancelled.
Trying to change the status of the contribution to either cancelled or failed produces the following error:

Cannot change contribution status from Partially paid to Cancelled.

If regular contributions can be cancelled, why can't partial ones?

This is reproducible on the demo site. Steps to reproduce:

1. Create a paid event with a single fee category.
2. Add event registration for any contact.
3. Leave "record payment" checkbox checked and enter an amount that's less than the fee category selected. Leave status dropdown to "completed".
4. The event status up at the top changes to "partially paid" but the payment status does not change.
5. Click save, behold your new partially paid contribution whose status cannot be changed.