Hi @nick ,
Judging by what is found online this seems to be an ongoing problem (not waking from sleep, that is) over the years with certain HP laser printers
If your previous firmware was working OK you may have to go back to it if you can.
Just curious what happens if you set the Auto power down delay to OFF (if possible) and then set a Wake Event. Does it turn the Auto Power down delay back on to the default value at all and does it alter anything?