Auto-lock on IPad not working with Kindle app.

I like to read using the Kindle app on my iPad while going to sleep. In the past, the auto-lock feature would shut down the iPad if the app was dormant for more than a couple of minutes (for example, after I fall asleep while reading). Now, I am waking up to find the app still running hours later. I haven’t changed the settings (other than by downloading the latest IOS). There is no setting to control this that I can see either in the app itself or in the IPad settings that are specific to the Kindle app. I’ve tried reinstalling the app but this has not helped.