If you don't have any problems with the stock firmware, it could be a bug or a wrong setting.I did reflash with all three files but I did not realise that the HEX file for options was not suitable - I sourced an S19 file and the reflash worked fine. I have since ridden the bike on standard firmware, which was trouble free. I decided to reflash with the OSF and it also worked fine. Fast forward two more rides I notice the assist degrades and disappears despite the battery being fine. A power off and on will fix this for a while....at which point the single bar of battery issue reappears and assist may or may not work. Another reflash with stock firmware and that problem disappears - difficult to see how this would be firmware driven behaviour.
Try posting the configuration ini file.