I believe the opposite is the case. It is not leftists moving from California to other states it is the republicans and conservatives.
For decades after WWII California drew millions of people from all over the country seeking better climate, jobs and affordable housing (which it was back in the good old days). Add to this the fact that California had one of the best public school systems in the country. A lot of these people who moved to California came from the northeast and most settled in and around the L.A. basin and the Bay Area. These people did not flee their old homes because they disliked their leftist state governments and brought their leftism with them. They and their descendants vote solidly for democrats.
It is also a fact that for decades “immigrants” have been settling by the millions in the state. The children and grandchildren of these people are now legal residents and tend to vote leftist because that is what their parents believed and voted for.
With the advent of leftist domination of the state conservatives have been leaving in droves to escape the high taxes and insane regulation. This only serves to make the remaining electorate more and more leftist as those who would vote otherwise flee the state.
The rise of leftists in other states is not due to Californians moving in. If anything the California transplants are more likely to be conservative. The leftists in these other states are either moving in from places like New York or New Jersey (where leftist governance is still popular) or are home grown by being under the constant influence of a degraded media and ideologically driven school systems.
You are very astute.
We fled California, specifically Orange County, in 1969. Almost all the leftists I met in Oregon and Washington States since that time came from places east of the Mississippi River.