This issue started with me last night. There is nothing I was doing that possibily could have caused the issue, but I think I fixed it. After trying all of the advice on this thread, aside from the factory reset, I booted up my phone in safe mode and the buttons worked no problem again. I deleted the two apps that I had most recently installed (PC optimum and Tim rewards) and rebooted the phone normally. Now it works. I tested it for a few minutes before posting this. You may need to delete apps one at a time and periodically reboot to test if the buttons are working again yet.
Zmax pro safe mode:
When phone is on, hold down power key until option box comes up.
Tap & hold “power off” until a prompt comes up asking to reboot in safe mode.
Press “ok”
To exit safe mode: use reboot option as usual