Lex V2 not invoking Lambda Code Hook after "Success" condition on Slot Elicitation

The "Code hook" is not invoking Lambda after a successful slot elicitation (even though Lex indicates Code Hook success) unless I choose to invoke Lambda at every turn. I'd like to limit the use of Lambda to utility functions and have conversation flow and logic in Lex as much as possible.

Where am I going wrong?

---Background---

I'm writing a simple Intent in Lex v2 to get a number of days to schedule a reminder at the date calculated from the duration from the current date. I'm eliciting the duration in a Slot and then storing the calculated date as a Session Variable.

I can get the Lambda code to invoke under the following conditions:

1. Fulfillment: When "Invoke Lambda" is checked - The is with the message text coming directly from Lambda (with Fulfillment state as "Fulfilled" and dialogAction:type as "Close") or by delegating the response (dialogAction:type as "Delegate") to the Fulfillment "Success Response" which references both duration Slot and calculated date Session Variable values
2. When the "Code hook - optional" is checked, calling the Lambda at each turn.

The issue with #1 is that waiting until Fulfillment makes the dialog too verbose for my taste (i.e. "What's the duration?" -> "So, {duration} days?" -> "OK, scheduled for [calculated date]" -> "Scheduling successful for [date]") as I'd like to use this in SMS functionality. The user doesn't get to confirm the date, only the duration. The date is what I'd like to be confirmed.

The issue with #2 is that my Lambda function has to be more sophisticated (check for conditions, errors, etc.) and I'd rather only pay for Lambda invocations that actually do something.

Ideally, I'd like this to work like it's pictured here - with the "Code hook" only being called AFTER slot value elicitation and prior to confirmation. So, that I get a more useful confirmation message (i.e a flow that's more like this "What's the duration" -> "You would like to schedule in {duration} days, which would be [calculated date] is this correct?"

I can get the appropriate "Success response" from the Code Hook, but there's zero evidence in CloudWatch that the Lambda function was ever called and the bot fails on fulfillment because the session variable - [calculated_time] - is never set by the Lambda function.