Bitbucket Pipelines + Azure: 'Passive mode refused' midway

https://stackoverflow.com/questions/47665860/bitbucket-pipeline-deploy-to-azure-app-service-using-ncftpput-fails-mid-way

I'm having the same problem as these users.

At random times in the ncftpput I will encounter 'Passive mode refused,' then the deployment fails.

I'm using the sample Azure + Pipelines setup, deploying to an Azure Web App.

Any suggestions on where to begin debugging this issue?