can't copy paste multiple measures

The score I've attached as several instruments which are invisible, and a visible piano line whose two staves both show G clef. If I select (with the mouse) the first measure, or the first 5 measures, I can press CMD-c, then use CMD-v to paste them into the 1st Flute staff (3rd from the top). This works correctly.
However, if I select the first 8 measures of the lower piano staff, and use CMD-c and CMD-v, the first 5 measures paste as all rests, but measures 6 through 8 paste correctly.
This looks like a bug. Or is it a feature I don't understand.
I'm using a nightly build cfbff1e

See also a discussion in https://musescore.org/en/node/136931