In the SpaceX report, it says that the NTO struck the valve at high speed and pressure, causing it to shatter. Before that happened, the NTO leaked though the valve into the helium tank slowly and did not cause a fire even though it was in contact with the valve for a much longer amount of time.
> Not hard to achieve during unscheduled disassembly.
I think the assumption is that by the time a RUD starts you no longer care about whether the various parts of your fuel system are going to explode, since the fuel is a far larger concern and it's going to go up anyways.
With that sort of thinking in mind no one would make any failsafes. "It's fucked anyway" is a constrictive and dangerous way of designing things, and likely to introduce even more failure points.
There’s really no point in designing additional failsafes into a launch vehicle when it disintegrates which can happen for a variety of reasons beyond anyone’s control — RUD is really just an euphemism for an explosion.
But the same report also notes:
"Nitrogen tetroxide with a water content in excess of 2.5 percent can accelerate titanium intergranular corrosion to the point of violent reaction."