@bigyihsuan what input mode are you in? In normal mode where notes can overwrite I actually can’t reproduce this. If you are in “text” mode where notes shift over to make room for notes, then I do see you can’t paste. There are more restrictions when in text mode; we assume lines are independent and that you wouldn’t in general want to paste notes that would cause things to your entire melody across your whole to shift in a potentially cascading overflow across all the lines and potentially have everything misaligned to your chords (misalignment can only occur within a line). I’m betting it’s related to this, though need to see if it’s a bug or not.
I’ll add some more context to this. I’ve found the following: If active voice is 1 and I have melody in voice 1 and I do a measure-level selection -> copy -> change active voice to 2 -> paste, the melody is pasted into voice 1.
But if instead I do a 2D drag of notes in the note staff (not a measure level selection) and do copy -> switch active voice -> paste, it pastes to the new active voice.
2.12.11 has new measure-level voice copy buttons that allow preserving or disregarding voice information on paste. This allows a voice to be copy/pasted to a different voice at the measure level. 2D drag select also allows pasting across voices.