To be honest, if you watched lots of anime, there are tons of WTF-ending.
It also could be the reason why people give much lower score for that particular anime.
Heck, even I hate when anime studio do that that I gave them 0.5 score lower in my list.
As for my side, I just realized people tend to give non-1st season score MUCH LOWER if they not faithful to the original despite being good as standalone series.
I mean, I know some studios just want to cash-grab with anime hype-popularity, but what with lower score in their list.
Im talking about:
1. The disappearance of Yuki Nagato
2. Psycho Pass second season
3. Soul Eater Not.
4. Darker than Black
Im aware of couple "cash-grab" 2nd seasons that are much different of original source
1. Tokyo Ghoul 2nd season (I havent watch but read about it, but season 1 was super popular)
2. Log Horizon (not that bad, but couldve been much more awesome)
There are couple of terrible 2nd season though. Ive seen couple of them and I wished they just wrapped it nicely in 1st season.