Hi,
There's really no need to post your requests multiple times in the forum. FYI, I have deleted your duplicate post.
As it seems there's an issue withthe Elastic api in some cases in both v5 and v9. Our devs are curently investigating the issue. Since we've stopped support for v5 at the end of January 2024 a fix will only be applied to v9.
As a workaround you can switch your sending method to smtp and use the Elastic Email smtp credentials that are visible in your Elastic Email account.