If whatever fiscal policy you have makes overall nominal spending go up (or down), that would have an impact on inflation, so the central bank will remove money from circulation (or add money to circulation) to counteract.

Thus neutralising any multiplier, you might naively expect.

(Also keep in mind that when you don't tax some money, it also gets spend or invested etc.)

Which is exactly the point. What's the "marginal tax rate" on low and middle income people of the existing benefits phase outs?

The >100% marginal rates that create actual cliffs are so patently absurd that no one can look at them and find any way to justify it, but even when the combined tax+phase out rates are in the neighborhood of 80-90% of marginal income, that's still not what we want, is it?