Mail: Spacebar preview shows incorrect thread message

When I preview a message in Mail with spacebar, and when that message is part of a thread, I often get to see another message than the one I have selected. Is there a logic behind this behaviour, or is this a bug? It is very confusing when searching for a particular point in a correspondence.

Is this perhaps because some messages shown in the thread are in another folder?

There are of course workarounds, but it would be good for my sanity to understand this.