Modifying some chord members' length restricts editing of others

When I create dotted-quarter chords and modify some of their notes to be half notes, then attempt to do the same with the other chord members, the earlier-modified notes switch back to dotted quarters. Anybody else encountered this? I made these examples in a new blank file, not any personalized template.
Step 3 - select and press numpad-7 again.png
Step 2 - select and press numpad-7.png
Step 1 - selection.png

Select the stem, not the note head, and extend them all in one go.