I have been in contact with Fenix support and discovered the core issue.
The Calibration XML file for the Fenix itself may not record a zero value being transmitted by the controller as zero. The fix (as it stands at the time of this post) is to navigate to the Calibration XML file at C:\ProgramData\Fenix\FenixSim A320\calibration.xml
Then edit the first 3 lines of each calibration setting (representing Max Rev/Idle Rev/Idle detents reading from top down) to exactly 0. Save the XML then start the Fenix again.
At this point the throttle will no longer jump below the Idle detent when the physical Bravo throttle axis is at the idle position.
I have advised the Fenix team of my dissatisfaction with this system logic and settings and am hoping they will work to resolve this issue in a future build.