Just a guess:
if the approval process is on the task's "complete" status, what would you think about just searching for projects where the task's actual completion date is in the past month?
i.e. when a task is completed, does this trigger the correct actual completion date? (is the actual completion date based off the approval date or the date they changed the status?)
If based off the former, I'd probably concentrate my efforts here -- in a project report, this would be an exists filter looking for tasks named "XYZ" whose actual completion date was in the past month.
If it looks like the actual completion date hack is risky or just doesn't work, I would probably focus more on exists statements on a task report that bring in the singular task and provide the project information you need. This assumes one task per project or one approval per month -- so also a bit hacky. Such statements are based on the ARVSTS object -- you could do a keyword search in the community -- also, here's the most recent post.
https://experienceleaguecommunities.adobe.com/t5/workfront-questions/i-need-help-with-a-document-rep...
If you let us know which approach you want to try, people can probably help you refine that filter further.
Also of note: for more EXISTS knowledge, we're holding a webinar at the end of the month:
https://events.teams.microsoft.com/event/2699d8d3-233a-4f57-92f1-0e8bfaa01f1f@fa7b1b5a-7b34-4387-94a...
