Understanding Callback URL validation for send OTP

Hello folks,
I'm using this as my callback URL - "https://XYZ.execute-api.ap-southeast-3.amazonaws.com/v1/verihub-test"

Now it works perfectly fine for SEND_SMS endpoint, but its throwing error for SEND_OTP endpoint -
{
"message": "Invalid payload: callback_url ensure this value has at least 1 characters",
"status": 400,
"code": 400
}

I dont understand whats the issue here.
Help is much appreciated.

For reference the sample request body looks like -

{
"msisdn": "62111122222",
"callback_url": "https://XYZ.execute-api.ap-southeast-3.amazonaws.com/v1/verihub-test",
"template": "My otp is - $OTP",
"otp": "1234"
}

Thanks,
Arnab