It stays logged in, and that's deliberate. Staying logged in on https://needforwincasino.net/  is a tick box at login, on by default. UK players get the same. It drops after thirty days of no use, so it won't sit there forever. A new device asks for the code again. And the app remembers the tab, so you land where you left. That's handy for a quick look. If you share the phone, untick it. Unticking costs you a code every time, so weigh it up. Nothing gets logged out in the background otherwise. Mine's a personal phone so it stays on. I've had no odd prompts either. It just works quietly. Check the box at login and you're set.

Staying logged in is normal on personal phones and it's a convenience, not a risk, as long as the phone has a passcode. Worth unticking it on a shared device. Sessions usually expire after a few weeks anyway. If you're unsure, log out manually after a session. It's one tap.