This issue was reported by several of my colleagues, and I checked it and can reproduce the bug.
Drag+middle mouse button is used for dragging your video in the Program monitor. When the zoom level is 100% or Fit it works alright, but when the image is zoomed in, the middle mouse button fails to grab the image on the first click but catches it on the second. If you change the zoom level again this behavior may repeat. I couldn't determine what exact zoom level causes it, but it's totally reproduciable. You can see it in this video, once the mouse pointer moves and the image stays in place - it's this bug.
-The mice buttons work properly
- It happens in any Premiere Pro where this feature with dragging is available
- Several users reported this issue so it's not machine-bound. Windows only.
- Happens with any media, on any sequence
Steps to reproduce: see the video, do the same - try dragging the video in the program monitor with the middle mouse button, changing the zoom levels with the scroll.