botbuilder-dotnet: Facebook channel getting flooded with "Failed to validate incoming Facebook message"

Bot Builder v4.0.1 .NET Core 2.0

I’ve setup the echo bot and a Facebook channel, published to Azure and sending/receiving messages and everything else works fine.

However, I get flooded with these FB channel warnings around every 2 minutes: “Failed to validate incoming Facebook message. Make sure your FacebookAppSecret is entered correctly.”

I’ve triple checked the facebook app id, secret, page id, page token.

I’m using a Facebook app in test mode if that matters. Any ideas?

Custom event log details:

Application version: Thu 07/12/2018 21:33:44.42 
UTC (factoryvm-az373) [Build 36484] [Repo Intercom] [Branch prod] [Commit c1c58d0f0027fbe311b8d18906c95f78d1cfbf2e] 
Bot ID: Bot
Channel ID: facebook
Client IP address: 0.0.0.0
Correlation ID: |/6wjT/qVSf8=.ddc61fad_
Country or region: Ireland
Device type: PC
Event count: 1,00
Event name: Facebook message failed
Event time: 2018-07-13 11:21:58
EventName: Facebook message failed
Message: Failed to validate incoming Facebook message. Make sure your FacebookAppSecret is entered correctly.
Operation ID: |/6wjT/qVSf8=.ddc61fad_
Operation name: POST /api/v1/bots/Bot
Operation parent ID: |/6wjT/qVSf8=.ddc61fad_
Role instance: RD0003FF74BC14
Role name: bc-facebook-weurope
SDK version: dotnet:2.6.4-28384