Bar lines on pitched staves do not span the entire staff when bars are spanned down to a percussion staff and hide empty staves is used.
Look at the attached file:
Notice that when the Snare drum line is *hidden, the timpani bar line only spans half the staff. The first instrument can be any pitched instrument while the last instrument can be any unpitched percussion staff. If you add another pitched instrument, which is hidden, to the bottom of the spanned staves (such as a Xylophone) the bars are displayed correctly. As long as the bottom spanned instrument is unpitched, (and there can be any number of pitched and unpitched instruments between the two) the bug is seen.
*Hidden - Hidden by using the Style->General... dialog. I have not been able to reproduce this if you make the instrument invisible in the "Instruments" dialog.