Glitched trophies are becoming increasingly common. The only thing more common is people assuming it’s glitched, when they’re actually doing something wrong. This is a common problem with Drained the Flood, since the trophy is not actually associated with the quest, but something that takes place after it.
To unlock Drained the Floor in Horizon Zero Dawn: The Frozen Wilds, start by going to the dam and doing the quest. Towards the end you’ll drain most of it, resulting in you gaining access to an additional room. This room can be accessed before or after turning the quest in, so don’t worry about missing out.
In this new room there will be a bunch of valves you need to interact with. Use your focus to reveal the direction and make it so you can get to the end. Once you figure out the correct order, you’ll unlock the trophy.
Please note, some people have noted you need to turn in the quest to unlock the trophy. So if you hadn’t done that, turn it in before worrying about it being glitched.
If you’re still having problems, check out the video below.