SYMPTOM:
A user has an exisitng subscribed report that gets sent to different users as shown in the image below.

The user adds a required prompt to an already subscribed report via DIstribution Services. The following error can be seen in the DSSErrors.log:
Sample Code/Error
2014-03-07 00:36:57.255-05:00 The user is required to specify an answer for this prompt. The prompt's designer specified that the prompt must be answered but did not supply a default answer.
2014-03-07 00:36:57.255-05:00 FAILED SUBSCRIPTION '681647 3/7/14 12:28:23 AM': Error Msg: Error in MSIJob Execution. Error Code 0x800437FD. Instance ID = 01472AB3401CFAA7DAF87EB47EEFCF0F, Contact (San Francisco Books Manager, 6BF7F8E94B825A4D5E5F158B6B41B75F), User (SFOManager, B6CC2CA24D83B68D80BFB4A86832FC0D). Stop in 'MSIDeliveryEngine::PreTransmissionNotification, Delivery Component of Distribution Service' <Scheduler Execution ID = E7EB42954E2F53EE8EA3ED93CE47EF09>
2014-03-07 00:36:57.255-05:00 FAILED SUBSCRIPTION '681647 3/7/14 12:28:23 AM': Error Msg: Error in MSIJob Execution. Error Code 0x800437FD. Instance ID = 01472AB3401CFAA7DAF87EB47EEFCF0F, Contact (San Francisco Books Manager, 6BF7F8E94B825A4D5E5F158B6B41B75F), User (SFOManager, B6CC2CA24D83B68D80BFB4A86832FC0D). Stop in 'MSIDeliveryEngine::PreTransmissionNotification, Delivery Component of Distribution Service' <Scheduler Execution ID = E7EB42954E2F53EE8EA3ED93CE47EF09>
2014-03-07 00:36:57.255-05:00 INVALIDATE SUBSCRIPTION: Distribution Service: MSIDeliveryEngine::PreTransmissionNotification,invalidate child subscription becuase required prompt doesn't have a default answer.<Subscription '681647 3/7/14 12:28:23 AM' (ID = 01472AB3401CFAA7DAF87EB47EEFCF0F), Contact 'San Francisco Books Manager' (ID = 6BF7F8E94B825A4D5E5F158B6B41B75F) Schedule Execution ID = E7EB42954E2F53EE8EA3ED93CE47EF09>
In Strategy Developer, when checking under Administration > Subscriptions, the subscriptions have been removed as shown below.

CAUSE:
This is a known issue in Strategy 9.x.
ACTION:
Strategy will continue to investigate and post updates as soon as they are available.
WORKAROUND:
To avoid subscriptions from being automatically deleted after a prompt is added to the subscribed report, modify the subscription by clicking the button below.

Modifying the subscription after a new prompt is added to the subscribed report will work as expected.