The process was highly politicized and one party and the one in power while the census was administered stood to gain from sowing doubt, fear and confusion. I think the actual results gathering was handled normally but the idea of course was to depress census response rates.
Motive isn’t evidence. I haven’t seen any serious criticism of data collection methods. Just doubting results based on “stood to gain” is silly unless there’s actual evidence.
In what world would there be a conspiracy to reduce california population by 200k?
But again, motive alone is empty. There is "plenty of reason" for me to do all kinds of terrible things that I consider abhorrent and would never act on.
There were lots of news stories of how it was changing wrt to people here illegally. The supreme court even stopped some of the challenge of it, right? The article doesn't break that part down at least from what I could search in it.