Well i don't even play ranked yet and i wonder why can Akali stay invisible even after going out of the circle? just makes no sense to me, and yeah i got stomped by her but i'm not complaining about her ability to burst down anyone fed or not, just that one ability which saves her ass every time.

Correct me if i'm wrong.

She does have a small delay of stealth shortly after she leaves Twilight Shroud. It's just an effect to make it viable. If it was like a portable brush, it would be difficult to use it for escapes because you will probably be walking near it and it'd be real easy to spot her the second she walks out. it gives her a bit of a chance. That's the only reason I can think of. I've wondered a similar thing whether it be a boundary/visual bug or what.

Oh ok, thanks. I just find it odd she's capable of entering battle and leaving it with nearly no drawbacks.

The old Twitch was capable of doing this. Start attacking, push Q, run and hope you don't get hit, sheathed for 50 seconds. Sharp can also do this although, he was nerfed recently so he can't get as far away. It's the way stealth works. Stealth champions trade a damaging spell for some utility that usually gives them a small buff as well.

Evelyn, Twitch, Talon, Shaco, Teemo, Rengar and Akali all have stealth, and most of theirs work the same way hers does.

This is honestly my least worry when having an Akali on the enemy team.

As a support main I just drop a pink ward when she tries to use it in a teamfight.
Every. Single. Time.

They hate that.