If you use Enter to open emails instead of using the Preview Pane, you'll find that messages don't automatically read all the time anymore. Sometimes, VoiceOver lands on a "message content scroll area", which holds the headers and message text. It then either reads nothing, or reads the text only after reading the sender's name followed by some clicks and the word "unknown" twice. If this happens, you also can't use regular arrow key navigation to get around the text, you must use VoiceOver commands. Finally, to even get to the text, you must interact with the scroll area. All this just to read or skim through a message gets to be very frustrating after a while.
Bug First Encountered
How often the bug occurs
If this happens, you can still read the email's text. To find it, interact with the scroll area, find the "text" item, interact, and use vo-arrow key commands to move.