cannot lengthen a measure in a two-voice staff

There is a bug in MuseScore 1.2 (and even worse behavior in the latest nightly r5471), illustrated in the two attached files. Suppose I have entered some music and want to lengthen a bar, adding more notes to fill out the longer bar. In the attached example, I initially enter two voices in two bars in 3/4 time, then attempt to change the first measure to 6/4. I first add an explicit 3/4 time signature to bar 2, then change the time signature in bar 1 to 6/4. I have no problem adding the extra 3 beats to voice 1, but when I switch to voice 2, the cursor moves from the 3rd note entered to the 2nd measure -- no way to enter the extra 3 beats (except through a workaround, like swapping voices 1 and 2 and then entering the extra notes to the swapped line 1, then swapping back). The attached files are as follows: testinsert.mscz has the two bars of 3/4 as originally entered; testinsert2.mscz shows the state of things after I've changed the time signature in bar 1 to 6/4 and added three quarter notes to voice 1. I can't do the same with voice 2.

Incidentally, one workaround I tried in the actual project I was working on was to open a new file, enter the two parts directly in a measure of 6/4, then copy/paste into the 6/4 bar of my original file. This worked eventually, but some extra notes appeared when I pasted, that I had to delete. I can't reproduce that problem in my two-bar example.

I'm running MuseScore 1.2 on a Mac, OS 10.6.8. As noted above, I also tried with nightly release 5471 -- then I could not add the extra beats to either voice.

Ron