I figured this out recently because i have started using the subscribed thread feature a few week earlier from now.

When you click to reply directly on a subscribed thread (list shown in usercp) .. the message does not contain the title of the thread.. and the message that gets posted misses that info..

I know this is not a very important thing to worry about.. but i post this here.. because of the different behaviour of replying from a post and replying a subscribed thread directly.. :)

Best regards.