As I was setting a TremoloPan preset on my H9, I found there is a problem with the pedal. When I change another effect to the tremolo, it seems that the effect doesn't start from the note I play, sometime there is a little delay, but sometime it goes well. As the effect is set to a square wave trem, it is a true disaster when it doesn't sync to the song's tempo.
You may have your Boss ES-8 send a CC to your H9 when you switch to the TremoloPan algorithm, and map this CC to "TAP". Hitting Tap once can restart the tremolo.