We say that $(a,b,c) \in \mathbb{N}^3$ is a Pythagorean triple if $a^2 + b^2 = c^2$. Is there a characterization of those Pythagorean triples $(a,b,c)$ for which $ab$ is a square-residue modulo $c^2$?

This is related to an open problem I've been working on.