It seems there is a restriction in the filters for all forms reports where you can not filter by end date. You can see that end date is a system variable, but it disappears in the list when trying to choose it for a filter.

Is it only possible to filter by start date.

The use case is that they want to build a monthly report, that includes processes which were approved/completed in that month, not ones that were just started.

I can't put a date field on the approval form, just because the approval step has been passed doesn't mean it was not in order to be sent back for revisions. This is why end date is so important to know when it was approved and everyone is done with their tasks.