It's not a bug, that's by design. The time of the first message in a conversation marks the start time of that thread/conversation. A thread is just a hierarchical tree view of individual messages. It's just a manner of presentation. The top/first message is the parent node of the tree, the main branch. Child nodes fall under the main branch, then grandchild nodes fall under the child nodes and so on. I understand you'd love the option to sort threads (in the messages pane) by the dates of their most recent member node/message, but this may not be possible to achieve while at the same time maintain the tree view since the dates of the member messages play a major role in building that hierarchy.

I see, thank you for your thorough explanation.

a) My original concern was that with Conversation plugin, when threaded view is disabled, it is even sorted by the first mail date. This should probably go to the plugin devs thought.

b) Do you think, it is possible to write a plugin to change the date to the date of the last mail?

You're right, the plugin's dev would be better equipped to explain why it's not done the way you'd wish. Remember, though, that doing so would mean changing the date of the first message in the conversation, and that would alter its historical significance. Its simply illogical. I'm guessing you'd again wish to have the message retain its original date when not in threaded/conversation view? That, seems like a request for two things that cancel each other out of existence whereby you can't have both.