If the device is less than a year old, it is still under warranty. Keep that in mind as you troubleshoot.
Since some apps (Netflix, for instance) keep logins, that would seem to indicate it's not a basic issue with the device. But more troubleshooting could still point to the device.
You didn't say what interval there is between occurrences. For example, if you stopped using the device at 1:00, would it exhibit the issue at 1:15? 1:30? 2:00? Or would it take longer, such as 6:00, 8:00, or 10:00?
While Roku devices don't really turn off, some do go into a low power/sleep mode. Check Settings > System > Power and see if there is a setting there that can disable low power/sleep/whatever mode.
One other thing. And I'm asking because I've actually seen this. It's behavior I'm not one to do, but apparently many users operate this way.
When you exit the app, how do you exit? Some like to use the Back button until they are on the main menu. Others simply press Home button to exit an app. I'm in the Home button camp. Some apps -- and I don't know which because I just don't do it this way, but have seen it with others -- don't simply exit via Back, but will also log out using this method. I'm not thinking this is the cause, because I don't think most apps work this way, but it is something to check. If you are a Home button exit person, this doesn't apply. And, in the meantime, become a Home button exit user. At least until troubleshooting is done.
If, to this point, nothing I've mentioned helps, then let's look at the individual apps. Pick one. Any one that has the issue; that is, doesn't keep your login. Say Disney+.
Remove the app. Reboot the Roku. Then, reinstall the app. Do not rearrange the order of those steps. It's remove, reboot, reinstall. Gotta be that order.
Now, log in. See if the issue returns. If not, do this for each of the problem apps. If it does return, we gotta think of something else. By then, perhaps, others with useful suggestions may come by and hit on something that helps.
Roku Community Streaming Expert
Note: I am not a Roku employee.
If this post solves your problem please help others find this answer and click "Accept as Solution."