I was hesitant to post in the Bug Forum, so if it's a bug after all, I will just make another topic.
I know that there's supposed an achievement with finding a worm in the Oofa Fruit, so you can feed it to your pets. It doesn't seem to really act like other food items, though.
There's no indication that it fills your pet's hunger. It does, but I had to check after to see if this was the case (I normally only feed one pet, so the fact that some of my other pets had 1/10 is only because of the fruit). Of course, it also doesn't let me feed it to my pet in the resort either. There's no indication on Account Search that your pet has eaten it like other foods. It also doesn't say that your pet has eaten it when you try to 'use it' like with other food items. It does show up on the list of food that your pet has eaten.
I don't know if it's special because it has that special page that tells you if you found a worm or not, so the fact that it doesn't acts like other food might be intentional? I don't know how old the achievement is, but I feel like it would have been brought up before.
I guess it could be classed as a bug, in a way. That item is old, its been onsite forever and predates all the changes made to how a pet is fed. Back in the day when you fed a pet it didn't tell you what its new hunger was, didn't let you know your pet had eaten it, etc.
As you surmised, the oofa fruit's programming is different because of the page that lets you know if you got an insect or not. As the site updated how things worked, it was never changed.
No one really remarked on it before because it always behaved that way. LOL kinda like an old typo that no one notices anymore. The programming should probably be updated and you could amke a bug report in the items category.
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I know it is somewhat different, but I think it should show up in account search at least as having been fed to your pet(s), since it shows up on the Foods Eaten pages.
It probably wouldn't hurt to make a bug post about this for some of this to get looked into. Like the account search thing, and also maybe an addition to the page with the worm and stuff about your pet's hunger level increasing or something.