Halo saved films operate by replaying the input data in the engine and letting everything come out the way it did the first time. It's all 100% deterministic and relies absolutely on being able to utilise old versions of the engine that matched what was used when the saved film data was created. Otherwise, the original events of the match and the recording go out of sync when the results of calculations - small or large - don't match. You'd get players not dying or walking into walls, the score going wrong, etc. etc.

I believe someone at 343 Industries said that they wanted the ability to update the MCC often and quickly. As a result this version compatibility could not be maintained, and the permanent saved films feature was dropped. It was suggested that players use the Xbox One DVR feature to permanently save footage of games. Also, any time there is a patch for the MCC, the recent films list will be wiped.

Take from that, what you will.