I'm not a Christian, but I have family members who are, and their kids are frequently told by other kids that God doesn't exist. They really aren't bothered by it.
I think that's the main thrust of it - from both perspectives. If Christian parents share their belief with their children that God exists, but then also fervently claim to believe in Santa, that's going to just confuse the child as to what personal belief and conviction (not to mention honesty and integrity) actually means, regardless of what faith/belief choices they make for themselves in later life.
Their parents insist that they completely believe in two omniscient men, omnipresent, miracle-working, who are as old as time (and commonly depicted with a long white beard), both associated with carpenters/carpentry in a big way, both live up in the north, both know if you've been good or bad and both want you to behave in a way that shows you want to please them. Then, eventually, those same parents admit to their children that one of those beliefs was just a fantasy but the other is nevertheless still a devout conviction.
We always told our DS that Santa was a character - like Peppa Pig or Thomas the Tank Engine etc. - about whom people made up fun stories, but we personally didn't believe he was actually real; HOWEVER, we made sure he knew that many (most?) children DID/DO believe that Santa is a real person/being - and it most certainly isn't up to us to tell other people that the objects of their beliefs are not real or that they should not believe in them.
He seems to have turned out just fine - enjoys Christmas very much as a really exciting, special time and has never for a moment tried to disabuse any other child of their belief, as they of course have every right to hold, that Santa is real.
Even aside of any religiously-motivated considerations, I know it's considered complete nasty-old-stick-in-the-mud territory, but I also hold with the 'it is actually a lie' perspective. I don't necessarily have an issue if parents tell very young children about Santa - just like they read them stories - and don't explicitly tell them that any of them are or aren't factually/historically true stories (not that young children really understand the difference anyway); but for me, the real turning point comes if, when children are older and they outright ask if Santa is real or not, instead of replying "No, he isn't real - it's just a fun children's story, but don't spoil it for anybody else", parents continue to insist that he is real and, in so doing, (imho) show a lack of respect and disregard for their children's developing reasoning skills, as well as demonstrating outright dishonesty and presenting it as truth - technically (albeit with the best, kindest motives) gaslighting them! 