We apologize for the delay in response and for the frustration this issue may have caused.
The test send feature uses the synchronous debug send API (the same API that’s invoked from the test send in portal). The regular send API which is documented is queued and processed asynchronously. The latency behavior between debug and normal send will not be the same due to this and some other implementation differences.
Please note, that send latency will vary based on load and we do not provide a guarantee on a delivery window. If you're experiencing extremely latency-sensitive, we recommend our direct send API.
We checked internally with the engineering team and this isn't a known issue. This issue requires deeper investigation. Please reach out to us at firstname.lastname@example.org with the subject ATTN: Grace and in the body of the email include your Azure Subscription ID, region, namespace, and hub are you using