I tried locking a folder in YacReader on the iPad. It asked me for a passcode; I made one. The folder then shows with a lock icon. So far, so good.

When I go to open that folder, it asks for a password - but it asks for my main iPad password, instead of the password I just created. That's the first issue.

The second issue is that when I've finished browsing that folder and exit it, it remains unlocked. I long press to lock it again, but only the option "unlock" is there. The folder is not locked though - I can go in and out of it at will.

The lock function uses iOS security if it is available, like many other apps, it is not the app but iOS asking to verify that it is you, if you cancel that dialog you should see YACReader asking for the in-app password you created. This also works with biometrics and face id.

Once you unlock your content the app knows it is you who is using it, so it will keep the content unlocked until you send the app to the background or you lock your device.