21 is quite high. In my shop that is wet / Green unusable honey. Like a green banana vs a yellow banana. Green tomato vs red tomato. etc.
If not that, then a walk about the forage area around the hive(s) can usually identify what plant(s) are or were yielding at the time that frame was filled. Not all honeys are pleasing to the palate. Some are actually quite nasty.
Likely none of that helps, but that is what came to mind ...
Hey, there's no bad ideas when you're brainstorming!
Based on the color, flavor, and time of year, I'm almost sure this is sourwood honey that something happened too, not a weird variety of honey I've never seen before.
I've been kicking around a couple of ideas, none of which I can really confirm or not.
1) This honey is fermenting, but something is different about how it's fermenting, because it doesn't smell, act, or taste like my typical fermentation. Could it be a different strain of yeast? Could the yeast be reacting differently to the high-fructose sourwood as opposed to the high-glucose honeys that typically ferment for me?
2) The bees got into something that they mixed with the sourwood, something that either was fermented and caused the honey to ferment as a result or something that is sweet and oily.
The thing that seems strange to me is that this frame was fully capped. So the bees must have been satisfied with the water content, which is odd. I mean I know the bees didn't read the book, but it makes me wonder if the water content was low when it was capped and it's high now BECAUSE it's fermenting, but why was it fermenting in the comb?