High impedance unity gain nJFET buffer
Well, a JFET connected as a current source is not an ideal current source. It might help to think of your JFET as a current limiter rather than a current source.

Background info:

An ideal current source will provide whatever voltage is needed to keep up the specified current, be it negative, positive, practical (mA, A) or impractical (MA, GA, TA) or insane (1e+3000&nbsp;A).

A JFET in a so-called "current-source connection" will limit the current in its branch to whatever value its saturation characteristics allow. It is, by itself, not a source that provides power, but a component that can only dissipate power (there's no built-in magic battery that will turn on when needed). Think of your JFET as a controlled resistor that will adjust its resistance such that it allows a somewhat stable current over a somewhat wide range of voltages across its drain-source terminals.

Calling a saturated JFET "current source" is industry jargon and inaccurate. It is assumed that some limitations are known, and the purpose of this jargon (slang) is to make things short. Stuff is getting lost along the way for the benefit of short explanations. In real life, there is no such thing as an ideal current source, so the inaccuracy won't hurt much unless you cross the boundary from practical circuits to academia. For practical purposes, current sources always have severe limitations and when designing a current source, you go great lengths to avoid them, or you know the limits. Strictly speaking, and within academic context, you are, however, correct to be confused ;-)