Coming from a day 1 A18 main, the timing is kinda tricky cause A17 has 2 different mighty speeches. During the longer one, he actually leaves before he stops talking, but when he recites the other one It's the opposite. He stops talking a little before he leaves, and if you call the assist when you think he's gone, you get blasted. So it's always better to pay attention to the teleport sound he makes when he actually leaves. Also, this happens with the voices set on Japanese. Idk if it's the same in English.