Mojave Mail conversation - messages are the same

Just updated to Mojave. In mail, conversations were including related messages so I unticked this in Preferences. I prefer just to see the thread below. When I click on a message heading on the left the message shows up on the right. However, when I click on any other message headings in the same conversation, it just shows the same message on the right as the first one I clicked. I have to click twice (and it opens in a separate window) to get the relevant message. Can you please help me sort this out?