I doubt it went under the barrier if it was installed deep enough. Barriers can be penetrated, so that is why I don't bother with them. All of my runners are contained by rhizome pruning once a year. It sounds like you're making a big problem out of a small problem. If the bamboo has gone where you don't want it to, chop off the offending rhizome and pull it out of the ground. Then continue to enjoy your bamboo, and practice rhizome pruning in the future.

Barriers are far from full proof it seems. Maybe the reduced maint was worth the barrier install and maybe not. I think the best place for a barrier is when bamboo is near a fence. It will minimize digging in the neighboors yard and it invariably comes up right under the fence where you cant dig. I say use the barrier as long as its in the ground. Prune the runner and trade it here. And Dig back to the barrier and see how it escaped and show us the pictures to see what the weak point was.

I agree with the previous posts. Barriers seem to not be fool proof. We had several rhizomes escape this spring after 5 years without problems. We were using 30mil, 24" barrier, now we switched to 40mil 30" on the new installations. Our soil is extremely hard clay, so you would expect bamboo to have a hard time escaping, but as they said in Jurassic Park: "Life will find a way"
We have a phyllostachis that escaped through the seam in the barrier, even though it overlaps 5 feet and was taped very well on both ends with double-sided tape. Once the first rhizome had breached that, several others followed.
We also have a pseudosasa japonica in another small grove. One rhizome of this one actually grew all the way down inside the barrier and came back up the other side. So even in hard, well compacted clay there's no guarantee the occasional rhizome won' make it through underneath.
Lessons learned:
- Use at least 30" tall barrier
- Use stainless steel clamps for the seam. Overlap doesn't do it.