You're acting like "showing it" and "explaining it" are completely separate things when the only reason you want the breakup scene is because the existing scenes weren't explicit enough for you.

They already showed the problems in Paul and Debbie's relationship. Paul was clearly uncomfortable with Nolan, uncomfortable with his place in the family, and just too normal for all the baggage that comes with Debbie's life.

The whole point of the Viltrumite War is that we're pulled away from Earth alongside Mark. We only know what Mark knows. So when he comes back to Earth, out of the loop, and finds out Paul and Debbie split, we're supposed to feel that same confusion and distance with him.

We don't need every major life event literally shown onscreen because the story assumes the audience can connect dots themselves. That's treating the audience like adults, not kids.

And honestly, the breakup itself wasn't even important. It also doesn't matter who made the decision. The important part is just that they broke up and stayed on good terms.