Why is AWS Billing failing with "there's no any timestamp column in header" after moving the Splunk App for AWS from the search head to the heavy forwarder?

We configured AWS billing on the aws app. Things worked fine until we moved the app from the search head to the heavy forwarder. Now we are seeing a lot of failures with the following error: File "/opt/splunk/etc/apps/Splunk_TA_aws/bin/aws_billing.py", line 896, in _stream_monthly_billing_item raise Exception("there's no any timestamp column in header") Exception: there's no any timestamp column in header I copied the inputs over from the working system so nothing should have changed. I do see the errors on existing files. So I'm not sure why it can't find the timestamp anymore.