Where I have a block of messages that are the result of a first message, and then back and forth correspondence using "Reply to", the content does not change as I select different emails. Where I have such a thread (all with the light blue colour), and I click on one email I can read the message. However, If I click on a different email in the same thread, the message remains unchanged. For example: I click on one and the message is "Hocus pocus blah blah blah". Then I click on any other email in the same thread and the message content will always be: "Hocus pocus blah blah blah". It does not matter which message in the thread is clicked on first.
In order to read all the messages I need to click on the first email, read and then click on any message not in the thread; Click on the next email, read and then click on any message not in the thread, et cetera.
I thought this might be an indexing issue but Cocktail cautions: Operation not permitted while System Integrity Protection is engaged. It provides instructions to disable and re-enable System Integrity Protection but I thought "I don't think I should goof with that".
Additional info: It seems to be where the emails have been filed. For example, it's not happening with threads in the Inbox.