Thanks for feedback.
My monitor is not G-Sync compatible, so I can only modify the VSync parameter. However, either activating VSync or disabling it through NVidia Control Panel or inside XPlane doesn't change the result on GTN flickering.
Yet, I noticed that the flickering is intimately linked to the FPS. By changing some graphic parameters in XPlane, I manged to lower the FPS to something close to 20 and to 30 as well (instead of 40 in the video). At 20 FPS (varying between 19 and 21 roughly), the flickering is almost gone. It's not resolved, but much much less frequent.
So it has indeed something to do with frames synchronization...
Moreover, interesting (or maybe not) fact is that in P3Dv4, with the f1 GTN, I don't have any flickering... But maybe it's no comparison as you don't use same technics to render, I don't know at all.