Thankfully, we're gonna be able to upload them to YouTube so they're saved forever. o3o

The reason they disappear is because the replays… well, literally replay the match. The inputs are recorded, not the match itself, as that would include any kind of lag involved, and when you choose a replay, the game's code recreates the match exactly how it played out.

With alterations to the game, data is overwritten and therefore clashes with the replay's data. It's literally impossible to get around this.

So, in other words, chill.