When you try to search for an User in the Workflow settings Apps, the search help doesnt respond any data
If the workflow which you are testing with refers to an outdated version of the workflow scenario, where the peoplepicker is not yet available, then the search help doesn´t show any valid CBuser.
If you wirte manually the CBuser it works correctly
When you create a new workflow by clicking on "Add" in the list view, the user validation/user search will work as expected.
Add a new workflow instead of using an existing workflow to copy
KBA , MM-FIO-PUR-REL , Fiori UI for Purchasing Approval , Problem