I'm having this issue with my custom integration to Salesforce from an Azure function right now. For some reason when I test locally and use the local endpoint with Postman I can authenticate with Salesforce just fine but when I publish my Azure function to the cloud and I swap out the endpoint my authentication fails for Salesforce. The exact error message being authentication failure.
At first I thought there must be something wrong with my IP settings for the connected app in Salesforce but I've tested setting Relax IP Settings as well as not defining a Trusted IP Range for OAuth Web server flow and still no success. Here are a couple of screenshots of my settings.
Any ideas why I can authenticate locally but not once I publish to my cloud service in Azure? (I'm using SalesForceSharp to authenticate if that makes any difference)