If you lose connection and your party completes the mission...

Does that mission count as completed?

It depends. If the party completes it while you're on the "do you want to reconnect" screen, it counts as completed. If you get botted back to the login screen, it won't.

Is this like a bug? Sounds really familar on some games.

I don't know for sure, but I wouldn't count on it. If it's just a PUG, you'll have to hope either they don't finish it before you get back, or they wait for you. If it's guildies/friends it would be best if they waited for you to return.

Simply put, if the game registers you as disconnecting without logging out, you have 10 minutes to return, during that time, any accomplishments, such as EotN reputation. mission / quest completion, etc count for your disconnected character. However, if the game registers you as logging out, then you are gone immediately and get nothing more.

I remeber getting d/c'ed during a mission once cus my internet is a big pile of fail, my party completed the mission though and i got credit

As long as you can use the reconnect then yes, it'll count.
If it takes you back to the log on screen or your chars it treats it the same as you logging yourself out.

i think so, it happened to me once, when i sucked and i couldnt ever beat the first kurzick mission, (mission 6) i was disconnectected and i was like frick, we were so close to winning too. and i reconnected, logged back in, and i had the mission completed master, i was like...cool, i dont have to do that again