Odd quirk (bug?) when unpowering a line of powered repeaters

I came across something recently and I'm not sure if it's a bug. If I have a line of repeaters set to four ticks each (powered by a torch), and then I send a four-tick pulse into the block the torch is on, so it loses power for that time, the repeaters will power off one after the other, with that pulse travelling down the line, BUT the final repeater never changes state. It doesn't seem to matter how many repeaters are in the line, the final one stays powered. Does anyone know why this would happen? If I use a piston pushing a redstone block powering the line, rather than a torch, I don't get the problem, so could it be to do with the delay caused by the torch?