That's a big drop while trying to do the right thing.
I've had unexplained "bad" tanks and only later realised that I mustn't have filled the tank completely at the start, or maybe overfilled when refilling.
Your post seems to say that you haven't actually filled the tank yet. Maybe your fuel gauge is playing up?
Lately, my fuel gauge drops like a rock, but when refilling, I find it's still a long way from actually empty.
Anyway, I hope it's nothing serious.
