I checked it again and found something interesting - there was a slur added to the Horn 3 part - but placed correctly in the condensed staff (Horn 4 voice).
I think this is either a bug, or a “feature not implemented yet”…?
After I did the manual condensing on the stem-up voice, I saw there was no slur on the Horn 4 notes - same as stated in posts above. So with condensing still on, I added a slur to the Horn 4 notes in Write mode by first clicking on the B-natural, then highlighting the slur button, then clicking again on the B-natural. Then I posted the screen shot.
A slur appears - perfectly lined up on the Horn 4 voice in the condensed score (the screen shot I posted earlier), but in the Horn 3 part there is now a new slur on the tied G. The Horn 4 part is untouched. I didn’t notice the slur in the Horn 3 part when I made my original post…
As DanielMuzMurray indicated above, if the custom scale of that slur is reduced to 1% in the part, it becomes invisible - So I guess that’s still the work around for your issue.
My original post was unfortunately misleading… I did add a slur, not to the Horn 3 part, but to the Horn 4 notes in the condensed staff, then discovered the new slur is actually visible (attached to?) in the Horn 3 part.
Is something in the manual condensing setup forcing the slur to be visible in the Horn 3 part, but perfectly placed in the Horn 4 voice on the condensed staff? Why would Dorico attach a slur to a tie chain if it considers tie chains to be one note? This is where I’m thinking “bug”…
Finally - I tried adding a slur to the tied G in the Horn 3 staff and Dorico would not let it appear. If this is the rule - it shouldn’t attach to a tie chain when condensing is on either… or am I missing something?
(Mac - Ventura, Dorico 5.1.70.2200)
Here’s the file I edited - showing the slur on the tied G in the Horn 3 part, and on the Horn 4 voice in the condensed staff (score):
Horns condensing example.dorico (3.1 MB)