present tense - ''When'' condition? When we've finished, we'll have

I had a question in an exam and I don't understand why I was wrong.

1) When we finish the painting we'll have a cup of tea.

I PUT THIS ANSWER AND WAS WRONG

2) When we've finished the painting, we'll have a cup of tea.
THIS WAS A GOOD ANSWER, BUT I DON'T KNOW WHY

Is this a when clause? Is it present perfect? What is the struture of this so I can search more for this type.

I have seen this sentence before but don't know why it is correct, can someone explain please?

Thanks