Oh you've got me good there. The problem is I read for meaning and context while you listen to your priest/bishop. Now if you would've taken the time to read the passage for yourself, you would have found out the leaven was the teachings and influence of these two influential Jewish groups, which Jesus found to be harmful and misleading. Leaven, or yeast, is used as a metaphor to represent how a small amount can spread and corrupt the whole.
Matthew 16:8 Jesus knew about their conversation and asked, “Why are you discussing among yourselves that you don’t have any bread? You have so little faith! 9 Don’t you understand yet? Don’t you remember the five loaves for the five thousand and how many baskets you filled? 10 Don’t you remember the seven loaves for the four thousand and how many large baskets you filled? 11 Why don’t you understand that I wasn’t talking to you about bread? Watch out for the yeast of the Pharisees and Sadducees!”
12 Then they understood that He didn’t say to watch out for the yeast in bread, but to watch out for the teachings of the Pharisees and Sadducees.
So, it's not speshul insight but it's taking the time to understand Scripture which you are lacking.
But the disciples understood the context in which Christ was speaking. (We have no disagreement here about what you have put here from Matthew.)
With regard to John 6, the question is why you don't take the words of Christ the way everyone described in the passage does.
(There is no similar passage in John 6 where "they" are recorded as understanding that Christ spoke figuratively and not literally. In fact, it's the opposite. John recorded that many left Him, and He simply watched them go.)