Never noticed this since my monitor has a sleep feature on it that I use while downloading contents to make sure a download doesn't get interrupted. Was a little confused at what you were saying because with the "Disable screensaver//standby/hibernation when downloading videos unchecked" the screen saver is supposed to be active and apply itself.
What got me now was you noted, it wasn't doing so after 1 minute after downloading and seating at idle.
I tried both settings in Streamfab for the screensaver, with it on and with it off and both with Windows screensaver set for 2 minutes of non-activity. The setting for the screensaver to sleep doesn't work while Streamfab is displayed, doesn't matter if it's downloading or not, off or on. Only when I shut Streamfab down did my screensaver come on. Appears to be an issue.