Specifically, WLPT gives higher CO2 readings than the older New European Driving Cycle (NEDC) did, and that is causing issues with ultralow emissions vehicle tax credits. Adjusting vehicles to fit the new testing method takes time and money, things that most car makers weren't prepared for.
One option to fix the underperforming PHEV's would be to fit larger battery packs to minimize the amount of time the vehicles are running on their internal combustion engines. Another would be to retune the internal combustion engines, but given how finely major automakers tune the cars from the start, there's likely not much left on the table when it comes to further emissions improvements.
Given the German auto industry's troubles of late with meeting the older emissions targets, we're guessing there are a lot of tense meetings going on in Wolfsburg, Munich and Stuttgart right now.
2018 Mercedes-Benz GLE550e is a plug-in hybrid family hauler