I've found out how to replicate the autobackstep problem, and an apparent (but annoying) solution. Let's say I have a bookmark set at 15 minutes into the file and the counter is at 0:00 when I leave the file. If I start playing the file from 0:00 autobackstep works. If I open the bookmark list and go to the previously set bookmark and start playing with the foot pedal, autobackstep doesn't work. If I immediately close ES and restart it, I'm still at the same point in the file, and autobackstep once again is working.
For me the problem seems to appear if I set a bookmark and then either continue playing the file, move to another part of the file, or jump to another file, autobackstep stops working. Restarting ES seems to fix the problem, but only until I once again set a bookmark and then continue playback of the file.
Hopefully this will give you enough information to replicate the problem at your end.
Tom