Comparator output stuck on

I'm making a Zombie Survival map, but I seem to be having a problem.
I'm not sure if this is a bug or this is how it's supposed to be, but either way this is a huge inconvenience.

I am using the /testfor command to test for a location of a player. The command block is activated by a trapped chest, so it isn't on a colck. If the player is in that location, the comparator sends a redstone signal. Once the player leaves, the signal stays on. In fact, the signal stays on all the time after it has been activated once. I believe it is a command block bug, and not a comparator bug, as when I turn off "previous output", the signal turns off. If I keep the previous output off, and open the chest, the bug happens again.

Here is a video of what I am experiencing

https://www.youtube.com/watch?v=azVfD_XX2lc

Sorry the video is so short, my internet takes forever to upload so I had to make it as quick as possible. If the video or description of my problem doesn't help you enough, I can explain it over skype

Also the contraption works as I intend if I run the command blocks on a clock