Meaning of ControlName index as used by Ndef (and NdefGui)

I’ve noticed that when you manipulate the sliders in a NdefGui the index printed by the control index changes to nil.

n = Ndef(\test, { arg amp = 0.3, freq = 888; amp * }).play
n.controlNames do: _.postln

// move the feq control in the gui, then
n.controlNames do: _.postln

would pring something like

ControlName  P 0 amp control 0.3
ControlName  P 1 freq control 888

for the initial values but e.g.

ControlName  P 0 amp control 0.3
ControlName  P nil freq control 1492.9520816834

after moving the freq slider. The same nil-ing of the ControlName index actually happens when you set a value programmatically e.g. n.set(\amp, 0.1) the printing the controls again would show ControlName P nil amp control 0.1.

What’s the meaning of this ControlName index change to nil? Is that how the proxy tracks that it’s not at the default value anymore or is it something else happening?