I'm having the same issue. I've had the Aquatic Life screensaver ever since I got my Roku Streaming Stick 4K back in December, it was in the list of my screensavers to choose from. Yesterday when my screensaver came on it was the City Stroll one.
I went to my settings and Aquatic Life was gone from the selection list. However if I scroll down further there is an option to install Aquatic Life as a channel on the home screen, this makes no sense! I do not want this as a channel button on my home screen I want it back in the list of screensavers so it just comes on and I don't have to install it as a channel.
I have two Roku Ultras the 2017 models and both of them still have Aquatic life as normal in my screensaver list WITHOUT having to install it separately as a channel on the home screen.
What is Roku doing? This makes absolutely NO SENSE at all! Fix it now!!!