Methane has more than 10 times the warming potential of carbon dioxide.
That means if the grazing plants capture 1 mol of carbon dioxide from the air, and then is consumed by cows who turn some of the carbon into methane, then the warming potential of the system has increased even though it's still carbon neutral.
Methane breaks down though, it only gets worse if we scale up production. Basically eating meat forever is equivalent to a year or so of other carbon pollutants. I'd rather we regulate other emissions a year or so early than ban all meat forever.
That means if the grazing plants capture 1 mol of carbon dioxide from the air, and then is consumed by cows who turn some of the carbon into methane, then the warming potential of the system has increased even though it's still carbon neutral.