On the gondola ride at the end of the mission Quarantine Zone, none of my Sangheili allies fight. They stand there with their weapons and don't react when the Flood attack them. This has happened to me every time since the game was released and I was wondering if this had been noted by 343i or if anyone else was having issues with it.
I have had that happen to me on single player. My advice, just avoid the fighting on the gondola ride by getting on top and waiting for the ride to end then use your camo to get to the next area. 343 probably has note of all the campaign bugs people have posted in these forums and others since November, but choose not to address it. Nothing you can do about them being lazy in fixing this games problems.