Not sure if it’s Mojave’s settings or a potential bug of 3.0.
After installing 3.0 Apple Mail plugin, I switched to the import side panel (View>Import) and chose Emails>Apple Mail. When I used quicklook (spacebar) to pop up a window and scrolling down the list of emails in a mailbox, I could only quicklook the content of the first selected message but none of the content of the remaining messages in the list was updated/refreshed even though the highlighted selection was scrolling down. This is a consistent behaviour and happened in all of the mail boxes. The behaviour of using quicklook for listed items in the main window is normal.