Cannot send SMS in Vietnam with registered Sender ID (Pinpoint)

Hello, I registered for a Sender ID in Vietnam in order to send one-time passcodes and transactional messages. My request was completed and AWS told me I can begin using my registered Sender ID. However, when I tried sending a one-time passcode with the Sender ID, my SMS failed to deliver.

I connected Pinpoint to Kinesis to see the event logs and found out that using the registered Sender ID resulted in an INVALID record status. Strangely, when I use any other random Sender ID, a message will be delivered (with the correct one-time password, but the overall content is modified), and the log in Kinesis shows a SUCCESSFUL record status.

I figured that when I use a non-registered Sender ID, my message is intercepted and modified by mobile operators. But I cannot understand why when using my registered Sender ID, nothing is delivered and the record status is INVALID. The Pinpoint documentation: https://docs.aws.amazon.com/pinpoint/latest/developerguide/event-streams-data-sms.html says that INVALID status means the destination phone number is invalid. But that is not correct because I can send SMS to that destination with any other Sender IDs; it only fails with my registered Sender ID.

Has anyone successfully registered for a Sender ID and has been able to use that registered ID? Any helps are greatly appreciated!