There seems to be bug, where current threads merged with old threads leave the old thread in its position in the thread list, instead of floating the it back up the thread list.
An example: I started a thread discussing the BBC's reversal of the decision against Naga Munchetty
This was then merged (no PM to explain/discuss this) with the original thread discussing The BBC's treatment of Naga Munchetty.
Unfortunately, as the original thread had received no replies in 12 hours or so, it was so far down the thread list, nobody is going to scroll down (until their fingers bleed) in order to find it.
I suggest either ensuring merging of threads bumps the original thread up taking the position of the one that was merged in, or not merging threads if the original topic has dropped far down the thread list. (Perhaps if the original topic hasn't received a reply in 6 hours it would be deemed old enough to leave alone)