Sounds like, for anyone who hasn't tried this game yet, the best thing to do is either download the full version straight away, or download the trial first, but don't play the trial with your main account. Use a dummy account, see if you like it, and if so, buy the full version before you play with your main account.
I'm sure this can be fixed with a patch, but until then, this is probably the best way to go.
Also, I know that some games (Worms, Hexic, Texas Hold'em, to name a few) save all their save info in your main profile file, not in a seperate save file. For these games, there is no way to get rid of the save data once it's in your profile. I'm not sure, but it sounds like this is the case for this game as well, since it's not showing up in the memory pane.
I kinda wish they'd all do it one way or the other. The one thing I really miss about Xbox 1 is that every game had its own save files, and you could move most of them freely from box to box. Now, almost everything is tied to your profile and/or console id, which pretty much makes a memory card useless (unless you don't have a HDD).