Total speculation:
It might be that Neuville's standing at Hyundai is slightly worsening. It seems that it's mostly Tanak's input being used in development in 2020 and the car is no longer "for Neuville only" as seemed to be the case with Paddon, Mikkelsen and Loeb. Compared to these 3 Neuville could argue that he was winning rallies from the start with 2017+ Hyundai so it should fit him. But when the current champion came and struggled they started to listen to him instead.

One thing is the mental pressure, another is if they changed some salary terms (Adamo has certainly shown he is flexible when it comes to contracts). Either of these or both could lead to him wanting to cut salary for Gilsoul.