Last time I checked Face Sets were preserved in a more or less
predictable way when modifying the mesh with dyntopo. As it looks that
in some problems this may cause bugs and you can't see or use face sets
when modifying the topology of the mesh whith dyntopo active, it is
probably better to reset them when going from dyntopo to mesh. This way
you know that you are always going to get a predictable face sets state.