In reply to:

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each station has only ONE frequency in the whole area...

When activating the AF debug mode, I see that the frequency/station change occurs each time any frequency other than the one I'm listening gets stronger.

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If you see other frequencies in AF Debug, then the station you're listening to (claims that it) has more than one frequency. Perhaps your radio station is broken - did you try with different broadcasters? Ideally, unrelated broadcasters - e.g. in Britain I'd compare a BBC station with an independent one.