They usually save, so I can go back to the game and they are there as they should be. However, every now and then I'll fire the game up only to notice that the scores have reverted to an earlier save-state :/

Does anyone know what could be causing this and how to stop it from happening, please?

It is very hard to score in this game, I never tried to play this game. Getting trophy is one of the hardest part in this game.
Unless you use a very easy method you are unable to get high score in this forum.