Single player, you resumed from a save? The replays can't handle that. Supposedly one can merge the partial replays, but so far nobody was able & willing to tell me how. (And a botched frankenfile disabled the game for me until I deleted it with a file manager.)

I've tried to Watch Replay a couple of times after a game, and in both instances the same thing happened: my units do absolutely nothing and I lose the game. But that's certainly not what happened 

LMAO Well, I did this:   

Thank you, that you opened an issue https://gitea.wildfiregames.com/0ad/0ad/issues/8705.

@Atrik @Vantha this might be of interest to you since you now fixing code around formations and groups.