Did some missions in "Invasion" were borked?

There was no way to get anything better then "B+" in some specific missions while other were "S+" easy. It wouldn't be a problem if two missions weren't tied to the rank. Which led to frustration, remaking and hoping not to get those missions. Annoying grinding.

Anyone encountered something like that? One of those missions is inside circle when you get attacked by a dozen Rek'Sais. That one where a horizontal wall in the upper part and circle doesn't move.