Thank you all for the replies. I guess flat surface it is. I wanted to avoid acquiring such piece as I don’t know what else I’d use it for and I don’t want to store more junk. I’ll figure it out.
Fritz, I am aware of how to plane correctly and how to edge joint boards. The hump in the middle is not the only symptom I’ve described: Not being able to plane parallel lines (plane will not cut down to the line in the middle) and having my wooden jointer, which I flattened with my #5, do the exact opposite (leaves a hollow). Resting the plane on a guaranteed flat surface and using a feeler gauge is not the only way to diagnose it.