Why does Spock come across as so vague and submissive throughout Star Trek V: The Final Frontier?

One of the frustrating aspects of Star Trek V: The Final Frontier is the vague and submissive conduct of Spock throughout, even taking into account the fact that Sybok is his half-brother. It really does seem borderline whether Spock should still be working on a starship at all.

I was wondering whether there was an out-of-universe explanation to this, such as Leonard Nimoy's attitude to the part at the time, or decisions made by the directors.