Little bubbles are always perfectly round, but not big bubbles. They're all kinds of shapes. Air is rising and falling and curling everywhere, and Bubble Thing makes this visible. Air eddying round a corner will sometimes twist a bubble into a giant corkscrew. Air spiraling up a courtyard will spiral up your bubbles.
Every street corner has different whirls.
It's pretty majestic. And they leap right over taxis. If your bubbles aren't rising, move around a bit. You'll easily find an updraft. I've sent bubbles thirty stories high this way.