Shadow Walk's visual effect, a red streak crossing the target, only appears upon placing the slow debuff on them. It does not appear on the next Shadow Walk strike if the debuff of the previous hit is still there.

This means the visual effect is currently bound to the debuff itself and only applies upon the debuff getting created. The visual effects should either be created on attack hit and be part of the ability's script file, or apply on debuff refresh as well, staying a part of the modifier script file.