I found a situation where the stop position does not prevent modification past the red line in insert mode. Let me explain using an example.
In this example, I wish to move the selected E out of the C/E chord into a single note between the C and the G (thus also moving the G one quarter note to the right). As I don’t want bar 2 to be affected, I put a stop line at the beginning of bar 2:
With Insert mode active, I select the E and then press Alt + Right Arrow to move it out of the chord while pushing everything else from that point up to the red line. Here is the result:
As one can see bar 2 was modified.
However, increasing the duration of E (Alt + Shift + Right Arrow) instead of moving it behaves as expected: