The "Way of the Long Death" monk has a feature called "Undead Fortitude":
Touch of Death
Starting when you choose this tradition at 3rd level, your study of death allows you to extract vitality from another creature as it nears its demise. When you reduce a creature within 5 feet of you to 0 hit points, you gain temporary hit points equal to your Wisdom modifier + your monk level (minimum of 1 temporary hit point).
(Bold added for emphasis). How does this work when a WotLD Monk reduces a zombie to 0 hit points and the blow is shrugged off by undead fortitude?
If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5+the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.
I could see this working one of two ways:
A zombie takes a hit, and drops to 0 hit points. Then Undead Fortitude kicks in and on a success, the zombie comes back to 1 HP. The zombie had been reduced to 0 hp, so ToD activates.
A zombie takes a hit, and makes a roll to see whether it drops to 0 or 1 HP. On a success, it drops to 1 HP and had never dropped to 0, therefore ToD does not activate.
Is there any official ruling on this case? If not, what is a reasonable interpretation of these rules?