I don't get that. I highlighted the second half of the measure with the clef tool, and that's where the clef-change took place.

Thank you for your quick response. It works for me if I create a new document, but not for the piece I'm currently working on. I was finally able to effect the mid-measure change with a weird workaround: I changed the clef at the beginning of an earlier measure, then selected the area from there up to where I really wanted the clef change, and then used the tool to change back to the original clef.