[Affecting Christmas] Execute detect not detecting reliably.

Summary:
We are running an execute detect in a command block every tick and it seems to be not running every tick. It happens unreliably, and for no apparent reason.

Impact:
This needs to be reliable for our christmas content to work. This effects our core mechanic.

Repro steps:
1. Make an execute detect command in a repeating command block that runs a say command
2. Give a pig speed and ride over the block you are detecting.

Observed behavior:
The say command isn't run.
Expected behavior:
The say command is run, you are over the block for more than 1/20th of a second.

Video: https://drive.google.com/open?id=0Bx5nD0d_h4p9cFVqM3R1aFU5MWs