Hello,
From what i read "bright-up" is gould-speak for a free running trace, so i wouldn't look there, your trace seems ok in absolute terms of brightness. I don't know if your scope is similar to mine, but in mine the delayed trace brightens up a segment of the main trace, and that trace is then seen "zoomed in" somewhere else. You have a knob that says: "pull to locate" on the delay section, have you tried it? To me it looks like that brightening-while-delayed function is not working properly on your scope, play with it in the different modes and post results, also try applying a signal to Z-modulation input. You have the full correct schematic? the one you posted has some signal pictures similar to what you were trying to show on the video.