Thanks everybody for your insight. As Marc pointed out, I was using Shift+Alt+arrow operation. I’m still learning the key commands…
And I was also not aware that the block chord with cross-staff notation belongs to either side of the staff; thus some notes don’t get pushed forward by alt-shift-arrow operation.
I tested again and confirmed that the program operates as intended.
If I move notes with Alt+arrow with insert mode off, the notes overwrite the adjacent notes. If I turn insert mode on, the adjacent notes are moved to where the moved notes were – nothing gets overwritten or deleted.
I found Shift+Alt+arrow command is powerful but is a dangerous tool in insert mode because it might break the vertical alignment of the latter notes (if there are multiple staff).
I should remind myself to always select all the staff in the system when I use Shift+Alt+arrow with insert mode.