Genesyscloud_messaging_facebook 400 on webhook validation

Just noticed that the Facebook messaging integration fails during automated deployment. The HCL block is identical to our staging config, but prod rejects it.

Error: 400 Bad Request. Message: “Invalid webhook URL format”. The URL is HTTPS and verified. Using provider 1.15.4 and TF 1.6.5. Is there a hidden validation rule for Facebook webhooks in the API?