I suspect this is an issue with a dependancy - Looks like in the config for the SDK you can set an optional _HttpHandler value which AWS describe as “Fetch in browser and Https in Nodejs.” - I’ve asked the app services development team to comment on why / if that might be passing the wrong content-type charset in the request.
If you can see a way to explicity set it to @aws-sdk/node-http-handler | AWS SDK for JavaScript v3 it might help.