What's more, this was a decade after the series ended. He may have not been able to deliver TOS-styled Spock, anymore. TAS certainly would seem to suggest that ...
Huh? I think Nimoy's very cool, deadpan performance in TAS is closer to his third-season interpretation of Spock than his later movie performances are. And the only TOS cast members who gave good performances in TAS were those with prior voice acting experience, like Doohan and Takei (and Nichols through her singing experience). The production and the recording process were so rushed that the actors didn't really get a lot of direction, so those without voice experience didn't have any guidance in how to do it effectively.
Of course, the whole point of TMP was that Spock underwent a fundamental change in his worldview. He initially tried to be more cold and emotionless than ever, but then realized that emotion had value and should be accepted. And in the final act of the movie and all his subsequent performances as Spock, Nimoy has played the character more expressively, as the more mature and serene version of Spock who manages his emotions but no longer fears or represses them. So it's not that he "forgot" how to play Spock. He evolved
Although it does seem to me that in his two most recent, post-retirement screen appearances in the Abrams films, he hasn't really been playing Spock at all; he's just come in and been Leonard Nimoy.