Cursory research would tell you that OpenBox has zero awareness of power events, so this can't be its fault.
This would have to be solved somewhere else, such as the trackers for LXDE or Debian itself. Having said that, fwiw, I too am on Debian 8/LXDE and have never seen this, even after much customisation of OB and power services. Maybe it's PC-specific, or maybe it's file corruption: check if /usr/bin/lxrandr even exists, and again, bug either Debian or LXDE if it does not (or restore a backup).
