The Problem With Toning

Given your answer it's quite possible that improper storage is/was the was the cause for accelerated toning. As for your other coins that you mentioned did not seem to exhibit accelerated toning - I suspect they do as well, but it hasn't gotten to the point that you noticed it. And this is quite normal as few do notice it. But that doesn't mean it didn't happen for if it happens to one it almost certainly happens to the others as well.

Regarding the choice to dip the coin or leave it - if it were me I'd just sell it as is. And then buy another more suitable to your taste. I say that because the it looks like the toning has advanced to the point that there is a good chance that damage has been done. This is by no means a certainty of course as many coins that looked this bad have been dipped and turned out to be gems. But an equal or larger number have turned out the opposite.