Board Game Arena

So I am currently #1 on the Euchre ELO leaderboard, but I didn't get the achievement for it.

I'm guessing this is because I didn't "earn" it. The previous leader was removed because they hadn't played in 60 days and I "assumed the throne" from 2nd.

Is this a bug? Or is it designed that I have to play to trigger the achievement?

You have to play another game.

It is a "feature" that you do not get the ELO achievement if the current leader falls back (by losing a game or in your case by dropping out of the ELO standings). If you play another game and you are still the leader after this game, then you recieve the ELO achievement trophy.

That's how it works for all elo achievements. They only get handed the moment you complete a game, while simultaneously holding the corresponding position.