Issue solved.
For your info and reference:
The issue was linked to the SmartTop module which failed (after 4 years of loyal service, 2-4 times a day).
Before going to the dealership in order to complain about the electric assist motor not working anymore and the impossibility to fully close the trunk, I tried to disable the SmartTop module (always disable SmartTop before going to the dealership). Strange enough I couldn't connect my PC to the module via the USB cable.
2 electrical failures at the same time ? Nah, I don't believe in such coincidence.
So I emailed SmartTop support and they indeed confirmed that a total module failure could impact the electric trunk closure. They then quickly diagnosed the issue with the module. They shipped me a new one for free. I installed it and voila, the eletric assist motor could close the trunk
