External monitor connected via usb-c causes mac to not show low battery notification
Whether or not this is something Apple ought to fix is entirely a matter of opinion. You can report the case to Apple as a bug and hope that they will change the way it currently works.

In the mean time you can install a third party app to monitor your battery that can alert you when the battery charge is low. For example you can install SlimBatteryMonitor, which you can get for free here:

http://www.orange-carb.org/SBM/