I did manage to get the book returned after many attempts. I'm not sure if signing into my library from within the app made any difference or not. I think there were failed attempts after signing as well. I have, in the last, returned library books from within Bluefire and Bookari, and there was no signing in involved there.
To me, it makes no sense for Overdrive to have technical questions about the app being directed to libraries. Maybe some feedback on the review page is in order.
Edited to add: I'm pleased with the improved performance of the app otherwise, it would just be nice to be able to contact support or provide feedback directly.