Sure they explained it - the paper was coated in fertilizer, and when the billionaire friend of M's walked up to it, the radio transmitter in his lapel pin set off the igniter in the metal strip
I may have fallen asleep watching the movie and woken back up during a James Bond movie. I'm not sure.
Philo's Law: To learn from your mistakes, you have to realize you're making mistakes.

The way I rationalized it was Dylan Rhodes had been planning every meticulous detail to this caper for a long time and as good a job as he did putting this team together, he couldn't leave to chance that EVERYTHING would fall into place on its own. Since he was FBI in the Vegas office (likely also part of his plan), he knew that this high profile case would fall right into his lap, therefore giving him the "power" to control what direction the investigation went in. First time watching the scene after they were arrested in Vegas, Dylan talking to the Horsemen felt like an interrogation. After watching it again, it felt more like a "yearly review" to see how his pawns were progressing.

My thing about that robbery, why even admit to it? Why leave the card and make it a part of a big show? You just stole 3 million dollar and got completely away with it!

The point of burning it was to make it look like they actually robbed the bank using magic.
kcgray.com

But what was the point of burning it? So the counterfeit would not get circulated?

Yeah that bothered me too.. Thought it was a pretty good explanation other than that

In the flashback we see the money catching fire without a cause.

Last movie watched:
House by the River
(5/10)

Not if a lighter was held up to the flash paper. But you're right, we don't know exactly how it was ignited.