Expressions Maps and mapped Dynamics data are always sent on channel 1 on a polyphonic staff.
The expressions are only applied to the notes on Voice 1, regardless of the voice to which they belong.
It would make sense for the expressions to send controllers and data on corresponding voice/channel on polyphonic staves as indicated in the score.
In their current state, Expression Maps and mapped Dynamics are not usable for polyphonic staves.