Ok, so lets go with the general agreement on this site that a flame should not appear. Why is one evident in many ascents from the Moon?

You were given a thorough explanation of this on page 2 of this thread. Re-read it.

The conspiracists said that no flame was visible when there should be one. I can't recall any debunker stating outright "There should not ever be a flame visible".

Read your own question: "When the LEMs were supposedly leaving the Moon, they should have produced a large bright exhaust flame from the rocket propellant. Instead, zero exhaust. (I have turned this one around and have found evidence of a flame on one ascent of the LEM... just to prove the sceptics wrong!)"

Am I the only one noticing how you contradicted yourself. "Instead, zero exhaust." Followed by: "I have turned this one around and have found evidence of a flame on one ascent of the LEM".

Can you not see this? Do you not realise how this makes no sense whatsoever?

Even if you did manage to catch some sceptics out once in a while, that does not prove the moon hoax conspiracy, or necessarily even strengthen your case.