Thanks for your reply!
It’s good to know that its not related to the ES-8 but is an issue in the OS of the H9.
I’ve currently copied the preset and changed the mix value in there but it would be much easier if I could send an extra CC message to set it.
Luckily the program change and expression CC message work together when I activate the patch on the ES-8 as I can use the expression pedal to control the H9 via midi.
I’m not sure what has changed in the OS area between firmware versions but something like this should just work.