This is, in fact, basic functionality that should work as expected. So something is amiss.
I don’t understand why the notes aren’t copying, but you should try using the system track to select a range of notes. It’ll also select the time signatures, which you need to have copied over as well as the notes.
UPDATE: So far I’ve been able to get SOME things pasted back in, but only through repeatedly using the filter
Filter>Time signature, copy and paste into new file correctly
Filter>Notes and chords, copy and paste into new file correctly
Filter>Slurs, copy and paste into new file correctly
Filter>All Dynamics, copy and paste into new file correctly
UPDATE 2: When I copy and paste large sections into the same file, it pastes it in the bottom two staves even if the top only is selected.
The voices are all wrong and there’s some unholy cross-staffing going on, it may have to do with that. [I wish I could actually see in the bottom strip what voice is being used because the “always on” colors with no identifiers are, in my opinion, useless. Like, what does blue mean? Is it in the right staff or cross staff? Up-stem voice 1 or Up-steam voice 2?]