Because Lisa and Brian leave The Ultimatum in Episode 2 after learning of Lisa's pregnancy, they don't go through the rest of the experiment with the other couples. They aren't even in the finale, when we see the other four couples all get engaged. They are, however, part of the reunion, and they seem to realize that they don't share a bond with the other couples because they didn't experience most of what the others did during the two trial run marriages in the experiment.

Neither Brian or Lisa even showed up to the dinner before their exit early in the series to explain why they planned to leave. After that, there was speculation among cast members about if Lisa's pregnancy was even real. As we see in the reunion, her pregnancy was real, as is the baby boy she and Brian now share.

However, the one thing they didn't walk away from The Ultimatum with was a new group of people to bond over shared experiences with. But that may be for the best, since Lisa had enough trouble with Riah just because Riah had a couple of meaningful conversations with Brian. Yeah, I'd say it's for the best.

The Ultimatum is now streaming on Netflix.