Sniper duel advancement can be achieved with trident, but the description implies only arrows may be used

Not fixed in 18w22a, I'm still able to reproduce this.

How is this not fixed, exactly?

Killed a skeleton from more than 50 blocks away using a trident, and got the advancement.

This is fixed in a very sneaky way ^[1]: The achievement is now "Kill a Skeleton from more than 50 meters away" - it doesn't say anymore with what weapon...
See https://crowdin.com/translate/minecraft/9389/enus-de#5171353

^[1] @unknown^{: I saw what you did there}

... I was already wondering why that string was reappearing. Cheaters!

_{I guess it's fine, it was just as hard killing that skeleton with a trident as killing it with a bow.}