as of 3rd March, in the {game}\Data\Royalty\Defs\RoyalTitles\RoyalTitles_Empire.xml file, each royal title has a section on food requirement (from Esquire upwards), that looks like the following:

<foodRequirement>
<minQuality>MealSimple</minQuality>
<allowedTypes>Processed, Liquor</allowedTypes>
<allowedDefs>
<li>Ambrosia</li>
<li>Milk</li>
<li>RawBerries</li>
<li>DBH_WaterBottle</li>
</allowedDefs>
</foodRequirement>

Adding in the DBH_WaterBottle line to each title with a food requirement fixes the "had to eat lower class food" debuff that you'll be getting by default. I'm sure there will be a less hackish official fix at some point, but in the meantime for those wanting a quick fix ....