I am having a similar problem, but it does not seem to equate to long idle periods, just happens randomly
This one has been occurring at random intervals too for the past couple of days, but before was only happening after long periods. I will test the screensaver, and have other information that I was asked to collect as well that I will forward on to Roku per request, when I arrive home from work later.
Regarding the screensaver, I would note that sometimes I get the clock as I expect it, and sometimes I just get "Roku" floating around. Not sure what causes it to pick up on the time sometimes and not others.