i don't think this is as much a bug as it is just poor anticipation of the usage. the way that multiple displays works now is that the "primary" display changes based on where you are working, it isn't permanently set anymore. the menu bar exists on each display separately and changes with the active display and window, but the dock stays where you put it. so if you want it to switch displays, you need to switch it manually each time. you can do so the same way as you would view the dock in a full screen app, by moving your pointer to the bottom and then swiping down more as if you're trying to go even further off the edge of the screen.
personally, i think i'd prefer to have a dock on each display, as i only ever use my mac with side by side displays, but it would get complicated for vertically stacked displays. probably would best be solved my adding some user options to dock behavior to allow for various display arrangements.