I have updates to share regarding issue TASKSNET-4118. We have made progress in our investigation and like to inform that, current login+password authentication cannot be considered as enterprise-level solution. We performed a thoughtful investigation and found that Project Online endpoint that we use to retrieve data from Project Online doesn’t support modern (non-legacy) authentication flows and uses old-style Sharepoint authentication. Maybe Microsoft will fix that issue in future.
On the other hand, Microsoft Project Desktop client can access Project Online using modern authorization, but it uses non-public API and we cannot reproduce the protocol yet. We plan to investigate that scheme further in one of future releases.