Voice 1 (from upper staff) has rests on lower staff

When I started inputting the notes from voice 3 (baritone) into the lower staff of this score, rests appeared that weren't there initially. I clicked one of the rests and saw that it was tagged as voice 1 (tenor 1), although voice 1 is on the upper staff. I was able to mark them invisible as a workaround so that I could get a clean pdf for printing, but is there something I did that caused the rests to show up in the first place? Or is this a bug?