This is intentional.
By asking this question, they know that immigrants- even legal ones, because they may be connected to or know those who aren't present legally- will be less likely to participate or answer honestly.
That will reduce numbers of people counted in the same areas that the GOP is unpopular.
The numbers that come from the census are generated once a decade and are used to apportion everything from federal dollars for aid programs to representation in the US House of Representatives.
This is simply one more step toward the ultimate goal of consolidation of Republican authority, working in step with gerrymandering and voter suppression to reduce minority participation in the political process.