Changing receive date of a contribution won't change the date of the contribution activity

To reproduce:

1. Create a contribution - an activity will be created automatically
2. update the contribution record, change the receive date
3. the corresponding activity will still have the old date

What seems to happen is that the field civicrm_activity.activity_date_time (where the activity date comes from) is populated upon creation of the contribution, but not changed when the contribution receive date is updated.