I tried the following: 
1) set up the Harmony Smart Hub with Remote Buddy exactly as per https://www.iospirit.com/blog/article/184/-/ 
2) select the activity by pressing the respective button (Music/TV/Movies) at the top of the remote 
3) press a button => connects and I can control Remote Buddy on the Mac 
4) press the "Off" button" => disconnects 
5) press any button => nothing happens 
6) select the activity by pressing the respective button (Music/TV/Movies) at the top of the remote 
7) press a button => connects and I can control Remote Buddy on the Mac

So it seems that, after pressing "Off", you need to select the Remote Buddy activity on the Harmony first for the Harmony Smart Hub to reconnect.

In my test, Remote Buddy was the only Device and Activity set up in the Harmony software.

Do you have any other devices set up in the Harmony software? If yes, which devices?

When you expect the Harmony to reconnect to your Mac: 
1) did you previously select the Remote Buddy Activity on your remote? 
2) is the Hub/"BD Remote Contol" listed as connected (connected devices have a menu entry in a bold font) in the OS X Bluetooth topbar menu? 
3) if it is listed as connected there, does or doesn't Remote Buddy list it in its topbar menu?

If I know more about your setup or usage pattern, I can again try to rebuild/reproduce it here.

Thanks, 
Felix