What if it has something to do with the multiple personalities theory for blackbeard?
Not sure if you'll get the reference but I vaguely recall something from Yu-Gi-Oh during the duelist kingdom arc when yugi was dueling Pegasus. So Pegasus was able to read yugis mind and predict his next move, similar to how shanks has top rate fs. So yugi and yami (or the Pharoah) switched back and forth so that only one of them looked at the cards and the other had no clue and was the target for the mind reading.
Now what if blackbeard is able to do something similar to negate fs?