If you look a little further into this what you’ll find is that the 50 Hz mains supply frequency is superimposed onto the HF feed to the lamp.
This is because it is in effect an amplitude modulation of the power rail to the oscillator (there is no reservoir capacitor to take it out) and so the 50 Hz is still present as an amplitude modulated HF feed.
The phosphor within the lamp envelope being of sufficiently long persistence takes most of this effect out, and is why when you turn such a light off there is a short afterglow as the phosphor darkens.
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