What is the letter showing? The rank you got in multi despite your fail or the rank of a score you got previously in solo mode that doesn't appear anymore?
I found one method to remove the incorrect letter grades. If you open the song in edit then leave the editor (without having to change anything) the letter grade will be removed, but only for failing grades that happened during multiplayer. Songs passed in multiplayer and songs that you've passed are unaffected (as they should).