New GitHub App webhook does not trigger scale-up lambda

[bob-ashforth-sp]

We have had an instance of the app running for months. I'm creating a new app (in a different account) to provision a larger instance type. With the app fully configured, webhook events are being delivered successfully, but the scale-up lambda never triggers.
I'm going through the control plane details to look for the cause, but if this pattern suggests any specific errors to any reader, the information would be appreciated.