This is not really a big deal, though. Companions come and go. Some last longer than others, but saying she's being "replaced after one season" doesn't seem fair to the actress. Everyone on the show gets replaced eventually, including the titular Doctor.
Christopher had a public falling out with the creators and writers, and has refused to return for any cameos or specials. The whole story has been covered in the news (google will tell you everything that's known, along with several rumors) but that's pretty much all you need to know.
He wasn't particularly beloved in his episodes, and whether that's due to writing or acting is a part of the dispute.
It didn't help his case that his replacement, David Tennant, gave the consensus favorite performance in the role since the revival.
I loved the chemistry between Millie and Gatwa in the Christmas special. Hopefully it carries through S14 and there's a good in-universe explanation for the departure.