Synth playback with LFOs seems to ignore "when"

I have a synth to which I have added a LFO to modulate frequency with a sine wave.
I play the synth at C4 for 2 seconds when the B button is pressed, and I supply 0 as the "when" parameter, with the intention that the oscillator produces a consistent signal from note to note.
However, I am finding I get different playback from note to note. It is as if the "when" parameter is ignored.

It's not a bug! The answer to this was to set the LFO's "retrigger" to true!