There is nothing in the documentation detailing the minimum legal value that the Prescaler can be set to. One user has tried to set it to "1".
Since the addition of the "Filter" code, the filter clock divider is running at 1/4 of the prescaler value. This implies a possible minimum prescaler value of 4 (or 5, 6, 7, 15, 16, 17 or whatever ???) for the filter to run appropriately.
What is a minimum value and what are the effects (if any) of low prescale values?
It looks like the main bit divider and filter clock prescaler can run in different "phases" to each other.