Flex duct shows up in some of the worst places and I have a total disdain for it anywhere. Under homes such as yours, it is subject to rodents and cats, if they can get under the floor. I have seen liters of kittens inside flex duct on the ground.
I recommend replacing the flex duct with the same size galvanized pipe, covered with sleeve insulation. It is amazing how much more air can be felt from the registers when the steel duct is used. Another alternative is to use manufactured rectangular duct with a short piece of pipe, vertically to connect the two ends.