The IFD is currently set up for AviCurve as the dimBus curve.  That's expecting there to be a dimming rheostat wired to the IFD itself, and it's expecting that rheostat to have a "DAY" detent which traditionally drops the dimming bus voltage to 0.00VDC.  The dimBus transition percentage is set to 10%.  What this means is that above 10% the photocell is in charge, if ambient lighting causes the photocell to drop below 10% it kicks over to the rheostat input.  Since we don't have a rheostat input here, it sees 0.00VDC so the IFD is jumping to "DAY" mode which = "Full Bright".  

This should be corrected by changing the dimBus transition % to 0% and changing the Curve to "Proportional".  Here are the applicable excerpts from the installation manual.

The dimBus Transition % sets the threshold where the aircraft dimming bus takes over from the photocell. Below this threshold, the aircraft dimmer controls the IFD5XX/4XX lighting. Both the Bezel and Display fields have a range of 0 to 100, and the factory default is 10. 

Note: If it is not desired to hand brightness control over to the dimming bus from the photocell at any point, set dimBus Transition % to 0 (zero). Doing so will prevent the scenario where, in increasingly darker environments (e.g. flying past sunset into dark night), the display automatically dims and dims and dims and then suddenly jumps to bright.