Linked clips with both tracks selected in the PRV Track list.
1. Notes disappear when lasso-selected note(s) to the left of them are edited or moved, even if moved farther to the left of the disappearing notes.
This sounds like Preferences > Editing > Drag and Drop > Replace Old with New-Delete Whole Measures is checked. But it's not!
2. Notes are duplicated when Velocity controllers for lasso-selected note(s) are adjusted in the Controller Pane.
#1 also occurs.
Sometimes it copies the note once for each controller adjustment.
Sometimes it copies the note & moves it to the left of the original note. Undo does not work here.
Sometimes split notes can't be separated.
Linked clips can make editing content easier over multiple tracks, but is more difficult in the current state. Sonar has similar behaviors.
My workaround is to 1. avoid selecting both tracks in the PRV, or 2. avoid using linked clips.
Selecting both linked tracks in the Track list allows you to see them in the main window.
Edited for clarity