Hey Crayox,
The image I have above is showing you’re original flag on the right and one where I imported it into 3Ds Max and adjusted the max distance when painting the weights.
When I test everything on my end I’m not seeing any issues with the asset you’ve provided aside from the painted max distance.
Placing the wind actor and changing it’s direction has worked for me as well. I’m not sure exactly what may be going on.
We now have 4.8 final release that is out. Have you tried in this version?
Also, do you have the Content Examples project from the Learn tab?
If so, we have the cloth test level. If you check the assets in this you can see that Wind Directional Actor will affect them as well. There is a wind directional actor placed in the corner at the first example (1.1). I placed your asset in this test level and everything worked right away for me as well.
Let me know what you find.
Tim