I had this problem on a TYPO3 12.4 website with powermail version 12.5
The problem was that when doing an XLS export from the powermail Module for a page in the backend, I got an empty Excelsheet eventhough this page contained mail records.
This page had mail records from 2 forms and even after selecting the right form in the Reduce to one form field and clicking on the Filter List button and exporting
again, the Excelsheet still contained no records.
Solution
The cause of the problem was that the hidden HTML field with id "export_fields" which contains a list of IDs of the form fields to be exported was not being updated to the correct value after changing the form in the Reduce to one form field.
The value of this field set on initial page load was always maintained and this is because the f:form.hidden ViewHelper by default always uses submitted values
as the value of the field unless the respectSubmittedDataValue argument is set to false.
So the solution is to patch the powermail extension and set respectSubmittedDataValue to false.
On how to easily create a patch for a Composer package, click hier.
After this, the export worked as expected.