Slurs disappear when I change voices

Hi there,
I'm making a score, and would like to collapse 2 parts into one, so I am selecting part 1, exchanging voices 1 and 2, copying part 2 then pasting onto part 1, then swapping voices 1 and 2 back so I have both lines on the same stave. So far so good, but I've just noticed that when I swap voices 1 and 2 on part 1, the slurs disappear on voice 1 and I have to put them back manually. I've attached a short snippet of the file I'm working on. The ties are unaffected. I am using the selection filter so I'm not picking up the dynamics from part 2, but it has no effect if I turn that back on - the slurs go when I exchange the voices.
Thanks for your help,
Jane
GIT commit: f51dc11
Voices problem.mscz