I can see this happen too: when I open the project, the organ has a 3-bar multirest in bar 5-7. But if I open the staff visibility settings in the system break at bar 5 and switch the vc/cb staff to show, and then back to hide, then the organ staff shows the notes of the cello/bass staff. But when I save and re-open the project, the 3-bar rest is back.
The organ/basso part starts a divisi in bar 8 where the organ part differs from the vc/cb. Divisi condensing is on.
In the system break, you set the upper (vc/cb) staff to hide, but that happens to be the staff that has the tasto solo notes. The organ staff is empty there, and therefore the 3 bar rest is actually correct. The actual glitch is that the 3 bars do appear after touching the divisi or visibility settings and changing them back. That’s unexpected and a small bug, I think.
If you want to show only the organ part on letter A, simply write the organ in the bass staff. Maybe condensing a 2-staff divisi is not the best approach here. You could use the trick to have a 1-staff divisi (i.e. a divisi with 1 staff only, not really dividing at all, just like you already do in bar 17), just to enable divisi labels.
If you also need a cello/bass part, just use the ‘legacy’ cello part that is still in your project. You might put cues from the cello part into in organ part where applicable.