In the Feynman Lectures, Chapter 21, I find the statement
We have solved Maxwell's equations. Given the currents and charges in any circumstance, we can find the potentials directly from these integrals and then differentiate and get the fields.
In Purcell's book on electricty and magnetism, I find the statement
Except for the possible addition of a constant field pervading all of space, the conditions $curl({\bf B}) =4\pi{\bf J}/c$ and $div({\bf B})=0$, uniquely determine the magnetic field of a given distribution of currents.
I don't have Griffiths's textbook in front of me at the moment but I'm pretty sure he says something similar.
Clearly all of these statements are false. For example, if the current and charge distributions are identically zero, then I can solve Maxwell's equations by setting ${\bf E}=grad(f)$ and ${\bf B}=grad(g)$ where $f$ and $g$ are arbitrary harmonic functions, so that in particular ${\bf E}$ and ${\bf B}$ are by no means unique (even up to the addition of a constant vector field).
Presumably, then, there is some hypothesis that Feynman, Purcell and others have omitted, possibly because they thought it was too obvious to mention. What is that hypothesis?