Hi nameless yes if the fillet handler was able to build fillet surfaces, but wasn't able to figure out how to trim away pieces of the original object to make space for the fillets, it will generate those surfaces. They're not curves, they're surfaces but they are sunk down into the object so all you see of them are the edges.

And yes it does that so you can have a chance at using them to still finish the model.

Sometimes a boolean may work with them but also sometimes you might need to do some manually trimming.

- Michael