This endpoint returns a 404 for an invalid or expired verificationId. It is also the case that you can only use a verificationId once for verification before it becomes stale.
What does this mean for your workflow? In my case, I was trying to verify a user's registration in the same request cycle as verifying their email. To do this, I needed to fetch the user response to get the user ID to verify the registration. Naturally, I was doing this after posting the email verification -- the result being said 404.
So, in terms of workflow, if you need to look up a user by verificationId, be sure to do it before you actually use the ID for verification.
Let me know if I got any of this wrong @arihantverma52