divisi voices cancel accidental when note is doubled

Hi all

Interesting problem here, see attached example.

These 2 measures should be the same. (note the A# on beat 4) In the second measure, I added a second voice, doubling the notes on beats. When I did this, the accidental for A# vanished.

Playback still sounds an A#.

Can anyone help?

Thanks so much

Dirck

Mac OS 10.6.5,
Musescore 0.9.6.3