The regulator is overheating and shutting itself down to prevent damage to the board. When using 12v input, you can't pull very much current from the 5v rail without it overheating (it;s a linear regulator - so that 7v times however much current you're drawing is dissipated as heat, and that little regulator (which doesn;t have the tab tied to a large copper area of the PCB to act as a heatsink like best practices suggest, even though components on the board could have been located soas to do that)