I have two Mibox S's and I have the same problem with both. As long as I use them on a regular basis there is no problem. But if I use the red button on the top of the remote to turn either box off, then if I don't turn it back on for a day or two, when I try to turn it back on with the remote it will not respond. I am not sure if it's the box or the remote, but the only way to get it to turn back on is to power cycle the box, then it turns right back on again.
I tried to troubleshoot both the remotes and the boxes with no luck. I tried re-pairing the remotes. I fiddled with screen saver settings. I tried turning CEC off then back on. I tried toggling the going to sleep setting off and on and nothing seems to help. The only thing I have not tried is a factory reset. But since it is happening on both of my boxes I don't think it's anything I am doing wrong. Both boxes have the latest updates. I did turn automatic updates off in an effort to stop Google Home from updating and displaying ads on the home screen, but this problem was happening before then.
Having the very same issue. If I don't use it for a while, it hangs and I need to power cycle.