Ms Project Online Custom Field does not rollup properly in some projects

In some projects, a task custom field that is formulated to rollup the value of [Baseline Duration] in summary tasks, shows an error when opening the project in Project Professional Desktop. The error disappears briefly when publishing and then reappears when closing and reopening the project. This does not happen in every project. It's very weird and the projects where the error is shown don't have something special, they are simple projects with baseline.