I have been getting logged out of the main Overdrive site frequently - for the last couple of months! I'll be right in the middle of doing something, having been active for the whole time, and boom - I'm logged out. This is not an inactivity timer. I get logged out mid-keystroke. Any given login seems to persist for maybe 15 minutes or so (I have not timed it). I receive no notification of the pending or completed logout. I just notice it when I click on a book and see that my local libraries are no longer in force (because I am logged out).
This is on the MAIN Overdrive website
https://overdrive.com, not from an individual library website.
From my computer web browser (not an app on my phone). I never stay on individual library Overdrive websites long enough to encounter an automated logout.
This has been going on for many months, getting close to a year now. It didn't used to be this way, once logged in I used to stayed logged in. But not any more.
OS: Linux. Web browser: Chromium