Rogue milestones on a timeline?

Yesterday I created a diagram with 4 cylindrical timelines, onto each of
which I placed/configured intervals and milestones.

Today I revised the diagram, dragging the intervals and milestones on each
timeline forward two weeks. But on one of the timelines, just the milestones
did not automatically update to their new date positions.

I confirmed that "automatially update" was checked for the timeline.

I tried reconfiguring one of the milestones, but got an alert message saying
I needed to drag it onto a timeline first. (It was already ON one.)

I tried to start fresh by dragging a new milestone onto the timeline, but as
soon as I configured it, it "jumped" up to land on the timeline above.

Eventually I just used "set milestone" to manually apply new dates to each
milestone on this one timeline.

Any ideas how to solve and/or prevent this happening again? Major PITA.

--
Eulilie