I used right clicks starting from the properly formatted page to navigate to a second tab also showing a age that wasn't displaying properly, but this time it displays properly.
To reiterate, one tab shows an improperly formatted version of an SI page, while the other shows a properly formatted version of the same page.
So I seem to have found a fix, assuming it doesn't recur. I will simply close the improperly formatted tabs and continue to use the properly formatted versions.
This still leaves the mystery of what happened and why...