I've seen this tendency fo the first time this year since I was trying to grow older heirloom bush beans. A lot of older varieties appear to make runners and will grow up if given support. Bountiful bush for example appears to be a half-runner. It latched onto taller plants nearby and hauled itself onto a neighboring empty trellis. However, the tallest vines on this one are only about chest height.

This being the case, do you like the flavor and texture of the beans your plant has produced so far? If it is just now making beans (my poles are just starting up) give it a chance and taste some. It could become your new favorite bean.

Never had a bush bean act like a pole bean. Half runners do to some extent. What "bush" variety did you intend to plant?

I too have never seen a bush bean turn into a pole unless it was a pole bean to start with. IMO, you got mislabeled seed.