This just started happening yesterday, I think -- recently, anyway.
If the last post in a thread is short, the red vertical progress thingy doesn't say n/n but rather n-1/n or n-2/n, and therefore this doesn't count as finishing reading. I can grab the red thingy and drag it down to n/n, but that doesn't stick and doesn't make the thread read.
The threads in question right now are HTML Blocks and Find Items in List.