In the attached file, there is a grace note run up to a regular note in bar 3 with an 8vb line covering those notes. If you move the 8vb line left by the rhythmic grid (select it and then type option-command-left arrow) it disappears unaffecting the regular note in bar 3 but still affects the grace notes. There is no way to get the line to reappear unless you have it still selected and move it back.
test.dorico.zip (263 KB)
When you hold down the Command (Ctrl on Windows) key as well as the Alt key to move the item, you’re moving it by the rhythmic grid value, and Dorico doesn’t snap it in the same way as it would if you were to use Alt+arrows on their own, or indeed to drag it with the mouse; because the octave line has a secondary offset at its beginning due to the grace notes, this doesn’t get changed unless you use Alt+left/right arrow. If you do this and you still have it selected, you can simply type Alt+left/right to snap to the next valid position, and it will reappear. You are a bit out of luck if you don’t notice, though.
Yes I understand what is happening and have worked around this. I was just reporting the display anomaly in case you thought it might be useful to fix one day.