I'm wondering if it's a case of classic voice stealing...
Check page 45 of the manual... Key Priority Modes. Hopefully it'll help make sense of the issue...
Thank you for your quick answer. It might be actually...
I did test with a patch with slow attack (both amp & filter)
Holding a 5-note chord, the 6th note is played with short attack once.
If I release this 6th note and play it again it will be played with slow attack just fine.
I've got the same behaviour in LAS and LAR modes.