Sice netuším, proč tě to tak vehementně zajímá:

NGen service allows the developer to defer the installation and update of native images to periods when the computer is idle.

The NGen queue supports a simple priority scheme to help establish the order in which applications are pre-compiled when submitting multiple jobs to the queue. The priorities also determine whether the service should wait for idle time on the machine to do the background compilation.
When the service detects that there are no more actions queued, it resets its status so that it will not restart the next time the computer is booted, and then it shuts itself down.

This means that this service will be shut down automatically if it finds that no action is needed. We have confirmed it with our product team. This behavior is by design.