Yamaha MM8 sustain issue

[Richard Olson]

I have a Yamaha MM8 and a PSR S700. When playing the S700 in layered mode, the strings sound will fade when the key is released whether you have the sustain pedal down or not. The MM8 will only release the string sound when the sustain pedal is released creating very muddy issues when the strings run over the top of each other.
Anyone know how to get the MM8 to behave like the PSR S700??