I dont know the answer,but cars usually have several drain pipes and holes.
Try a bit of experimentation in dry weather. Use a watering can to pour some water into the flap area (with fuel cap on
) to see if it drains through where described.
If it does thats probably as intended. And would only be on this side. If the draining seems to be slower than you would expect its possible the drain hole /pipe is partially clogged with dirt, dead leaves etc . You may be able to clear this with some careful poking with a bit of wire etc or squirting water.
You could also test other areas such as boot lid join, and roof channelling. I'd expect these areas to drain on both sides of the car .