Currently Alteryx does not support writing to SharePoint document libraries.
However there are success sometimes but not at other times.
Please see attachment where we ran into an issue.
See this link for additional information.
https://community.alteryx.com/t5/Data-Sources/Connect-to-and-Publish-Back-to-SharePoint-Online/m-p/4...
We need official support for reading and writing to SharePoint document libraries.
It's an important Output target, and will becoming more so, as Alteryx enhances its reporting capabilities.