This has never really worked for me.
Observing the behavior of CircleCI, it seems that autoscrolling enables whenever the bottom line of the current build step is somewhere near the bottom of the screen (autoscroll will start when you scroll near the end of the screen, just not all the way to the end).
The buttons appear to scroll to just that position for the step to which they are attached. That is, only the button for the currently running step will enable autoscroll – the others will just scroll the last lines of that step’s screen output into view.
If a CircleCI script has many short steps, clicking the right button becomes a game of whack-a-mole. Also, the button doesn’t seem to work when the current step is generating output at a high rate of lines per second.
So far, my best bet is to scroll down manually, hoping to somehow reach the position at which autoscroll will start.
I’d suggest adding an autoscroll button to the top of the build log.