If you’re asking when was it damaged, who knows. All that matters now is, is that it’s damaged and always will be.
Not during the minting of the Quarter! There are so many ways coins can be altered/damaged after it leaves the US Mint. And it could occur anytime after that
1991 Quarter damaged when? It happened when the person that damage it, did it. Only the person that did it, or caused it to happen, knows for sure, but as mentioned, not a mint error, just PMD.