I just set my Garmin 595 up on the workbench, in the Motorcycle cradle.
With the 595 switched off I applied 12 volts to the Cradle wiring,
mimicking turning on the Ignition - the 595 powered up.
I then removed power from the cradle, mimicking Cranking of the engine.
The 595 remained switched on, running off its own battery.
After a couple of seconds the screen displayed a message :
"External Power has been lost.
Turning off in 15" - and counted down in seconds of time.
There was an option to "Cancel" which would have cancelled the
turning off operation, and left the 595 running on it's battery.
The following short video clip shows what "should" happen.
I suspect your 595 Battery was not fully charged

