@FioreFire @JamesHenderson1065
It's not that hard for whole societies to develop misconceptions about certain foods:
https://www.smithsonianmag.com/arts-culture/why-the-tomato-was-feared-in-europe-for-more-than-200-years-863735/
Moreover, if you look at foods around the world, you'll see that some cultures have ended up eating some very strange things, like say Balut.
If the common knowledge is that green is unripe, red is ripe and beyond that is rotten it's hardly that surprising that they wouldn't want to eat kukuberries when they're actually ripe. Moreover, if they have plenty of other fruits to eat, they'll just never have any reason to try these.
What's odder in my mind is the fact that no one with the appraisal skill actually checked such things as when a food is edible or best to eat; but then appraisal is a rare skill and whoever has it is tasked with appraising people (as they said earlier, it's not even actually possible to appraise every person working in the castle so the skill is clearly rare and used only in certain instances.) In this instance, the MC has the skill, the cooking knowledge and the leisure to appraise fruits.
Of course, it's always odd for isekai societies to be so lacking in certain aspects, but that's just what needs to be done for your average person to turn into a superhero in another world.