When reporting problems it's always a good idea to report your Roku model and model number as shown via Settings > System > About. Often the answers you seek depend on your Roku model and if we have this information to start with, you can get targeted suggestions more quickly.
The Roku "Bandwidth saver" function kicks in when the Roku has received no user input in four hours. It asks if you are still watching, and if it receives no response, it exits the current stream and returns the Roku to its home page. You can turn this feature on and off via Settings > Network > Bandwidth saver. Note that some individual channels may have a similar feature.
Another possibility involves a recent Roku software update. What Roku software version is your Roku running? You can see that at the Settings > System > About screen.
Roku 10.5 is currently rolling out. I don't have this yet. If you have received it, some Roku models reportedly have had a new Power item added under the Settings > System menu path. If you have this Power item, go there and see if you can deactivate the idle timer.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X