>> Are you sure? Won't the sugar just dissolve into the water that is part of the yogurt? Or if a granule texture remained, just mix in sugar syrup?

Nope. The fluid you're trying to mix the syrup in is milk, already a stable emulsion of proteins, sugars and water. Try mixing sugar (well, corn) syrup in that and see how long it stays mixed in. Once you stop mixing and the mixture comes to rest, the syrup starts sinking to the bottom. Better drink fast, and it sure sets faster than it takes for yogurt to set.

Check out the ingredients on any chocolate milk. There's always a stabiliser, usually a caragenaan. That's to keep the corn syrup and chocolate powder from separating from the milk. Back Home in the Old Country (when I was a kid in Greece) the stores sold a chocolate milk which was just milk with cocoa and some sugar. When you bought it from the store you could see that the milk and solids had separated, and there was a darker layer at the bottom, where all the chocolate and sugar had set. So you had to shake it well before drinking. Nobody makes that anymore, now all the chocolate milks have a stabiliser, so you don't need to expend your precious energy shaking the bottle. All those wasted calories. We don't want that. I guess.

For similar reasons, when you mix honey in your tea you need to keep a spoon in to stir it often, or it all goes to the bottom.

>> I've definitely mixed honey into yogurt and put it in the fridge and it was fine the next day, no separation or anything. Why do you think you'd need a stabilizer?

Because I know yogurt. I basically eat yogurt every day. I even make it myself some times (see my recipe above). Are you sure your yogurt did not have stabilisiers in it?

Honey contains oligosaccharides that act as stabilizers.