If your Roku is running OS 9.2 or later, it has a feature called Bandwidth Saver. It is turned ON by default.
How does Bandwidth Saver work? If you haven't used the remote in 4 hours, a message will display asking if you're still watching. Choose "Keep Streaming" to continue playing, or "Don't ask again for this channel" if you want to turn off this feature for Tubi. If there is no response the channel will stop streaming and return you to the homescreen.
If you want to shut the feature off entirely (which will allow your Roku to stream any channel, including Tubi, for longer than 4 hours without interruption) go to your Roku home screen, then Settings > Network > Bandwidth Saver and uncheck the box.