The light turning off happens because of the Device Connection Manager in the EV3 firmware. The algorithm used to detect when a motor is connected relies on there being a load on pins 5 and 6 of the output ports to determine if the motor is a Large or Medium motor. The 8528 adapter cable only has wires on pins 1 and 2.
When you use a VM command (via a programming block or by using the on-brick app) to tell a motor to run, it forces the motor to run, but then the device connection manager immediately resets everything because it doesn't think that a motor is connected.
Also see How to use Power Functions with Mindstorms EV3?
The fact that setting a power of -100 keeps the power on continuously, though, is new to me. I'm at a bit of a loss to explain that observation.
I've done some digging and now I better understand what is going on.
Output port pin 2 is connected to pin 6 via a 100k resistor as seen in the schematic diagrams below.
When the EV3 is instructed to run a motor in the forward direction pin 1 will have battery voltage and pin 2 will be at 0V. When running in reverse, however, pin 1 will be at 0V and pin 2 will be at battery voltage.
When using the converter cable, nothing is connected to pin 6 externally, so it will also be at battery voltage (pulled down slightly by R216 to +5V). This triggers an event in the device connection manager. Most of the time (but not 100% of the time) the EV3 will think that it has a motor attached because of this and so the LED will stay on.