Reference Planes for Impedance Controlled Signals

I might be showing my ignorance by asking this question.
I thought I knew, but now I'm being challenged on it.

If I have impedance controlled signals that are part of
a circuit that is being run on +2.6V or +3.3V, but there
is a closer plane available, like +5V, the signal returns
will snap to the closer one, right? Does it matter which
TYPE of plane is being used a reference? The text I
received from a colleague implies that if my 2.6V plane
is too far from my 4th signal layer, which is next to the
+5V plane, then they will not be impedance controlled.
Doesn't sound right to me...

Jack (the new guy)

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