Thank you for your reply!
Let's make it really simple.
I created two staves in Sibelius, this time two clarinets, with the matrix Clarinet Bb+ from the Special Edition.
The two clarinets are on channel 1 and channel 2.
Case 1.
Both staves have one quarter note marked fp (not sfz this time).
Then I do the semitone up and pitch bend down trick on the lower staff.
When I play the upper staff the fp articulation is trigged and the note is played right.
When I play the lower (the pitch bend) staff the fp articulation is switched on right again, BUT the note keeps playing without stop.
Case 2.
I put a normal sustain note after the fp note on the lower staff (the pitch bend staff).
Then I play the lower staff.
The fp note plays as described above: the articulation switches on, but the note keeps playing.
As for the other note, the articulation is switched correctly to "sustain" and the note is played right.
But the fp note sounds without stop in the background.
Best, Pekay