Erratic seq rythm with pads & mode simult MPC60

Hi everybody
mpc60 MK1 os 3.10 Marion SCSI
I have a sequence with track 1 : kick ( playing all 4 beat) ;track 2 : snare ; track 3 : hh ; track 7 : bass ( playing 1/16) ... all quantized 1/16
I play the seq everything is tight .
Now i add 2 sounds (mode simult ) to the kick
and 1 or 2 (mode simult ) to the Snare
the rythm of the bass become erratic , not tight
if i mute the tracks of the kick and snare or if i change the mode of the 2 pads from simult to normal ,or if i solo the bass track , the rythm of the bass track become again normal .

does i miss something ?
Thanks

i think it's a kind of polyphony problem like here
https://www.mpc-forums.com/viewtopic.php?f=4&t=148351
or
https://www.gearslutz.com/board/showpost.php?p=12042864&postcount=129
if i play a sequence with kick on 1/4 & bass making 1/16 notes we can hear that , and with the pad of the kick playing simultaneous 2 pads it's more important , i have recorded the midi bass seq in my daw when the program of sound is out or the bass track in solo: notes are tight & on the time
and when the programm of sound is on : each time the kick or the snare play the bass midi note is played later , i believe that others elements that are playing ( hh percs ) in the mpc60 are doing the same thing , so the groove of the midi bass seq is influenced by the drums midi tracks , it's not a problem with minimal events but with dance beats & arp bass i can hear it . I have tried to shift bass midi track or kick and it works really better . Sorry for my english i do my best