Can reproduce this as described. Interestingly, the "site preferences" page zoom level stays at 100%, when I'd have thought it would have increased...
On latest beta, for good measure. On the first attempt, it works correctly, as I zoom using "vertical up/down two finger scrolling". On the second attempt, I zoom using pinch in/out diagonally, which exhibits the wrong behaviour.
Also, the zooming performance in the former scenario looks a little bit stuttery compared with the smoothness I'd have expected (the video is an accurate representation of the experience, and I was zooming nice and smoothly).