Note that they specify shortest trig pulse 24ns. So trying to trig on 1-20ns pulse lengths means you will test it outside the specified capabilities.
and that's the crux, hardware glitches are (most?) often shorter than 24ns.

I recall spending a month and a half together with a guy I consider extremely qualified chasing a hardware glitch that was shorter than the scope (of that day) could catch.

when considering scope specs, considering scoping normal events mkes no sense, why would you scope them if they were normal.

Mahmood,
just for kicks, why do you not make a "trigger expander" get a very fast retriggerable one shot, set it for ~33ns output and connect it to the external trig input and see if that works as a "trigger expander" you can make that while waiting for delivery and play with it and your current scope. If it works, it may be valuable for other people with scopes with subpar triggers.

Erik