Why do my time signatures look funny?

I pulled in an MS2 score into MS 3.6 (by default; turns out the behavior is the same if I pull the score into MS2). It's a lead sheet in 2/2, with the occasional bar of 3/2. By request I went to add a piano part. A lot of the 3/2s and 2/2s in the original remained so in the vocal line; but they became 3/4s and 2/4s in the piano line; and I can't change them. What happened here; and how do I resolve it?