Relevant rules text clarifying the mana ability nature of Grinning Ignus:
Although Grinning Ignus's ability has a timing restriction, it's still a mana ability. It doesn't use the stack and it can't be responded to.
From https://gatherer.wizards.com/pages/Card/Details.aspx?multiverseid=136040
Example of the bug occurring:
The discussion of the bug starts at the original timestamp, and then the bug actually occurs at 2:10:00.
There are many people on Magic Online that are aware of the bug and knowingly exploit it, and it has personally cost me 2 challenge runs and numerous league...