Since unlocking the Cochabamba region, I have only been offered La Paz and Cochabamba expeditions, at a ratio of about 80% La Paz to 20% Cochabamba. My preferred region was set to Beni before unlocking the new region. I have tried changing the region to La Paz and back to Beni, and resetting my expedition choices, but it did not help.

Update: after 30+ La Paz / Cochabamba offers, I've now received two Beni expedition offers.