Yeah, he was like that in person as well. When I was an undergraduate, he would "teach" a seminar on Tuesday afternoons called "PhysX" where you could go and ask any question you wanted. He'd go up to the blackboard and extemporaneously write things down and explain things in such a way that thought you really understood. But when you got back to your room and tried to replicate the chain of reasoning, there were always pieces missing or leaps that you now couldn't make. (It felt like the Star Trek Episode, "Spock's Brain".)
But we all took that as an indication of our own lack of knowledge and intuition and would just try harder.
I had a teacher in the University who took some courses taught by Feynman and had the same experience. He even tried to record some of the lectures, but the result was similar. While he was listening, he felt like everything was very clear. But as soon as he stopped the tape, nothing made sense anymore.
The funny thing is, if I remember his autobiographies correctly, he wouldn't let anyone else get away with that. I think he said that if he didn't understand something, he would always ask about it.
I also had a prof who took Feynman’s classes. He told us Feynman was a tough grader - he didn’t give partial credits, you either solved a problem or you didn’t.
But we all took that as an indication of our own lack of knowledge and intuition and would just try harder.