Dual boot OS X and Manjaro - Problems
My guess would be that OS X is clobbering your GrUB when it sets its own bootloader when you make the change in System Preferences. My advice would be to not set OS X or Monjaro as your default in OS X, but rather use GrUB. You can then set GrUB to default to OS X after a customizable delay, which I assume is the behavior you're looking for. Potentially helpful link