Isotopes and elements are human made labels based on nucleon and proton counts. Hydrogen and deuterium are both the same element, but are obviously different nuclear structures. Even at a chemical level deuterium is appreciably different. That difference between isotopes and elements diminishes but never disappears as atomic mass rises.

Isotopes are only imperfect in the context of one labeling system. But not from a quantum viewpoint.

The isotopes are examples of why atoms are not really atomic at all.

Once something that you thought was atomic actually shows variation you begin to suspect it is composed of smaller things.