Only for emergency landings. Airplanes are only fueled for a planned flight plus some reserve for economic reasons (carrying excess fuel means you consume more fuel and that costs airlines money). If you're making a regular planned approach you've already used most of planned fuel and airplane is ready to land as is. For emergency landings soon after takeoff, landing at that time would mean airplane is too heavy and would probably destroy landing gear and damage fuselage/wings on touchdown and you don't want that when you already have other problems. Only then they dump the excess fuel to get to the right landing load (weight). Source: I watch a lot of air crash investigations on TV.

My understanding…

You’ve misunderstood.

The only time fuel is dumped by aircraft is during an emergency and either 1) needs to land somewhere unexpected and is far overweight for the runway, or 2) there is a situation where the landing is exceedingly dangerous and there's a need to reduce weight and potential fireball size (e.g. landing gear failures).

Yeah pretty sure that's not a thing.