There is a 49.9 Ohm smt resistor connecting in parallel in 50 Ohm mode (bottom right in the attached picture), so you can burn the resistor if the voltage too high. The attenuators are behind that resistor, so it always takes the first hit regardless of attenuator setting.
EDIT: and in case you have not figured out yet - this piece of shi!t does not remember its 50 Ohm setting - you enable 50 Ohm input but next morning you power the scope up it may not be on 50 Ohm, even the channel setting says it is. So if you willing to take an advice - always cycle the channel's 50 Ohm setting when you need it to avoid nasty surprises.