API-created Demo envelopes show “Sent Invitations” but recipient email is not delivered

We have isolated a recipient email-delivery issue in our DocuSign Demo account.

Manual envelopes created in the DocuSign UI deliver recipient invitation emails successfully, but envelopes created through the eSignature REST API do not, even when sent to the exact same recipient from the same Demo account.

API-created envelope:

• Status: sent
• deliveryMethod: email
• Standard remote signer
• No clientUserId
• No embedded-signing configuration
• No email-suppression flags
• Envelope History shows Sent Invitations
• No bounce, rejection, or autoresponded status
• Envelope appears normally in DocuSign and can be signed successfully
• Recipient invitation email never arrives

Control tests:

• Manual envelope from the same Demo account to the same Yahoo recipient delivered immediately.
• Separate manual envelope to Gmail also delivered immediately.

Controlled comparison:
We have specific envelope IDs available for both the failed API-created envelope and the successful manually created envelope, but I have omitted them from this public post. I can provide them privately to DocuSign staff if there is a secure channel available.

We also verified that the account's embedded-signer email settings are irrelevant because this API recipient is a standard remote signer with no clientUserId.

I found several recent Community reports describing very similar Demo/Sandbox API email-delivery behavior.

Is there any known Demo/API-specific behavior, throttling, fraud/spam protection, or other email-delivery restriction that could cause DocuSign to record Sent Invitations without actually delivering the recipient invitation email, while manually created envelopes from the same Demo account continue to deliver normally?

If investigation of the affected envelope ID is required, what is the appropriate secure escalation path for a free Developer/Demo account so that DocuSign staff can inspect it privately?