QGIS novice here. I have a shapefile for US census PUMA (public use microdata areas). One of the attributes in my shapefile is the metropolitan area of the PUMA. I want to combine PUMA's which are located in the same metropolitan area. To do this, I am using the dissolve command, and choosing the metropolitan area attribute as the dissolve field. For most of the United States, things work as expected, but there are a few areas in which there is weird behaviour. For example, instead of combining the Chicago pumas into a big polygon, QGIS redraws a Chicago polygon of slightly different size. See the following pictures to see what I mean:
Is this a QGIS bug, or is there something I am doing wrong? Is there anything I might try to fix this behaviour?