Something strange happened this week when I was giving a PowerPivot training.
The PowerPivot Field list disappeared and the PivotTable Field List came back. This happened after a few minutes after I had made a PowerPivot table in a new document. It seemed to happen just in a moment, not after any particular action. Saving, closing and reopening the document solved the problem.
This triggered me to look at the autosave options. When you create a new document and autosave is on, make sure it saves it with the right extension. In this case, the format of autosave was on OpenDocument Spreadsheet (.ods). This caused the PowerPivot Field list to disappear and only the PivotTable field list was available.
This can be undone when the document type of autosave is on Excel Workbook.
The autosave options can be changed under File > Options > Save.