Hokay!
NORMAL when plugging in a 50-amp plug into a 30-amp receptacle is to lose one leg.
WHY? Think about it. Powering 2 50-amp rated circuits with 30-amps worth of power in the supply line "Leaves a 70-amp 120 volts shortfall. Very similar to like plugging an eleven hundred watt microwave using an 18-gauge power wire.
An Ambulance chasing tort mongerer would sweat and get giddy at the thought of suing once the fire is put out. Another way of putting it is to have a long length of 1/2" tow chain with one 3/16ths link.
This is why you cannot find such a coupling adapter off the shelf. You can wire one yourself and I know the recipe but I will not share it. Literally and fugurativelly such a device is a fire waiting to happen.
Yes the connection can be made up but so can a few quarts of nitroglycerin be juggled for sport.