I've been thinking about this, and I don't
know the answer, but since no one else seems to know either, here are my thoughts on it. Maybe it'll help, Zweef. I have no idea if I'm actually right or not, this is just my brainstorming on it.
Honey occasionally ferments in the hive, due to the bees not being able to cap it for some reason or pests like hive beetles opening up the cells. From my experience and what I've read, capped honey is pretty safe from fermentation. But in this situation we are asking what happens when bees attempt to process already fermented honey. Yeasts are always present in small amounts in honey, but they can't reproduce in low moisture, and some can't reproduce without air, which is one reason why the bees get the moisture content of the nectar down to ~18% and then cap it. When active, the yeasts ingest water and sugar and produce alcohols and carbon dioxide. Both of these product substances are volatile, so they should evaporate just like water when the bees fan and work the honey. As the bees process the fermented honey, my assumption is that, depending on all sorts of factors, if the bees can get the fermented honey down to ~18% and cap it, and if they have plenty of non-fermented honey in the hive to eat as well, it shouldn't be that big of a problem.