@15Redstones generally an interesting thing to think about.
Cases of golden flames as punishment, what the reader would put into context with a magic contract, are limited right now.
The commoner in P4V9 comes to mind who presumably was forbidden to disclose secrets and got burned right before he tried to snitch.
But there is a problem. The punishment for speaking the god of darkness and lights names is that the observer burns and the speaker looses their blessing (I assume power that would be granted by the knowledge of the names).
Meaning there is precedent for punishment before and after commiting the violation of the contract.
As a result of that I think we can only judge contracts that clearly specify when and what punishment would be delivered.
Benno has violated in some way the contract with Myne in the past, but only if he had refused to pay after being reminded to pay would the punishment follow.
In that sense I would be inclined to believe, that the simpler the conditions are the more powerfull the punishment would be.
Tell no one of this -> burst into flames imidiately
Tell no one our names (but well I can only try to whisper so much you know) -> Ok only the idiot who desperatly tries to eavesdrop burns, but you were too careless
Pay me (well you didn´t specify a timelimit) -> burn if you refuse
Don´t be my enemy (well you will be killed if I don´t interfere) -> ok only blatantly obvious actions get the axe
In general "a wizard did it" still applies to magic contracts I think. Whatever plays well with the story that Kazuki thinks up goes.
So a "Hey Wilfried remember the contract for the RMCM? Don´t become my enemy? Please for your own sake be carefull." would be hella funny and a loophole if a retainer of Wilfried has not signed the contract, because the thoughtless intention of opposing Rozemyne could bleed into that retainer without Wilfried actively instigating the murder so to speak.