Mail Bug: Wrong email shown in long thread (not organized by conversation)

Mail 11.5; macOS 10.13.6

This has bothered me for a long time: Having set the view in a mailbox folder to "Don't organize messages by conversation in this mailbox", Mail seems to have a bug.

When there is long email thread, about 30 mails or more, Mail stops showing the correct mail content when clicking on a particular email in the thread. Instead, one and the same email remains to be shown in the detail pane.

For example: An email thread with 30 mails #1-#30: I click on mail #5, content is shown. If I click on #10, #20 or any other in the thread, #5 remains shown. The only way to see a particular email's content, is to click another thread's email, and then click back into the thread on exactly the email you need to read (which then again remains the one selected).

It seems that the bug is really related to the number of emails in a thread. With a smaller amount of emails in a thread, things work as expected; in the same mailbox.

This is a locally stored mailbox "On My Mac", not a live IMAP mailbox.

Any idea how to fix this?

According to my enabled preference "Highlight messages with color when not grouped", the highlighting does work correctly. So I don't think this is an index issue or such; besides, I don't think there is a way to rebuild local mailboxes, or is there?

Thanks