That's where the ABV should kick in to maintain the minimum flow rate. Slightly different problem to the one I described, which is inherent in any cold zone flushing water to the boiler return (this could be because DHW kicks in, instead of another radiator zone).

A LLH would (hopefully) aid in both instances, as the hot and cold water from different zones should dilute at a slower pace in the header, allowing the boiler to sense the return temp dropping and have enough time to modulate down.