After some wild tests, it appears that if a character on the map is set with Bicep/Forearm or hand 215 from CEP3 (in appearance setting, not armors)… This happens.
As I can’t get rid of 215, I decided to overwrite it with a different armor set (all races, both sex) in my haks.
→ problem solved.
Just in case someone come across that problem, but I guess this was really specific to my case with both my haks/cep3 compatibility.